Replace mkfiles.pl with a CMake build system.

This brings various concrete advantages over the previous system:

 - consistent support for out-of-tree builds on all platforms

 - more thorough support for Visual Studio IDE project files

 - support for Ninja-based builds, which is particularly useful on
   Windows where the alternative nmake has no parallel option

 - a really simple set of build instructions that work the same way on
   all the major platforms (look how much shorter README is!)

 - better decoupling of the project configuration from the toolchain
   configuration, so that my Windows cross-building doesn't need
   (much) special treatment in CMakeLists.txt

 - configure-time tests on Windows as well as Linux, so that a lot of
   ad-hoc #ifdefs second-guessing a particular feature's presence from
   the compiler version can now be replaced by tests of the feature
   itself

Also some longer-term software-engineering advantages:

 - other people have actually heard of CMake, so they'll be able to
   produce patches to the new build setup more easily

 - unlike the old mkfiles.pl, CMake is not my personal problem to
   maintain

 - most importantly, mkfiles.pl was just a horrible pile of
   unmaintainable cruft, which even I found it painful to make changes
   to or to use, and desperately needed throwing in the bin. I've
   already thrown away all the variants of it I had in other projects
   of mine, and was only delaying this one so we could make the 0.75
   release branch first.

This change comes with a noticeable build-level restructuring. The
previous Recipe worked by compiling every object file exactly once,
and then making each executable by linking a precisely specified
subset of the same object files. But in CMake, that's not the natural
way to work - if you write the obvious command that puts the same
source file into two executable targets, CMake generates a makefile
that compiles it once per target. That can be an advantage, because it
gives you the freedom to compile it differently in each case (e.g.
with a #define telling it which program it's part of). But in a
project that has many executable targets and had carefully contrived
to _never_ need to build any module more than once, all it does is
bloat the build time pointlessly!

To avoid slowing down the build by a large factor, I've put most of
the modules of the code base into a collection of static libraries
organised vaguely thematically (SSH, other backends, crypto, network,
...). That means all those modules can still be compiled just once
each, because once each library is built it's reused unchanged for all
the executable targets.

One upside of this library-based structure is that now I don't have to
manually specify exactly which objects go into which programs any more
- it's enough to specify which libraries are needed, and the linker
will figure out the fine detail automatically. So there's less
maintenance to do in CMakeLists.txt when the source code changes.

But that reorganisation also adds fragility, because of the trad Unix
linker semantics of walking along the library list once each, so that
cyclic references between your libraries will provoke link errors. The
current setup builds successfully, but I suspect it only just manages
it.

(In particular, I've found that MinGW is the most finicky on this
score of the Windows compilers I've tried building with. So I've
included a MinGW test build in the new-look Buildscr, because
otherwise I think there'd be a significant risk of introducing
MinGW-only build failures due to library search order, which wasn't a
risk in the previous library-free build organisation.)

In the longer term I hope to be able to reduce the risk of that, via
gradual reorganisation (in particular, breaking up too-monolithic
modules, to reduce the risk of knock-on references when you included a
module for function A and it also contains function B with an
unsatisfied dependency you didn't really need). Ideally I want to
reach a state in which the libraries all have sensibly described
purposes, a clearly documented (partial) order in which they're
permitted to depend on each other, and a specification of what stubs
you have to put where if you're leaving one of them out (e.g.
nocrypto) and what callbacks you have to define in your non-library
objects to satisfy dependencies from things low in the stack (e.g.
out_of_memory()).

One thing that's gone completely missing in this migration,
unfortunately, is the unfinished MacOS port linked against Quartz GTK.
That's because it turned out that I can't currently build it myself,
on my own Mac: my previous installation of GTK had bit-rotted as a
side effect of an Xcode upgrade, and I haven't yet been able to
persuade jhbuild to make me a new one. So I can't even build the MacOS
port with the _old_ makefiles, and hence, I have no way of checking
that the new ones also work. I hope to bring that port back to life at
some point, but I don't want it to block the rest of this change.

doc/udp.but

@@ -333,45 +333,6 @@ on a 640\u00D7{x}480 display. If you're adding controls to either of
 these boxes and you find yourself wanting to increase the size of
 the whole box, \e{don't}. Split it into more panels instead.
 
-\H{udp-makefiles-auto} Automatically generated \cw{Makefile}s
-
-PuTTY is intended to compile on multiple platforms, and with
-multiple compilers. It would be horrifying to try to maintain a
-single \cw{Makefile} which handled all possible situations, and just
-as painful to try to directly maintain a set of matching
-\cw{Makefile}s for each different compilation environment.
-
-Therefore, we have moved the problem up by one level. In the PuTTY
-source archive is a file called \c{Recipe}, which lists which source
-files combine to produce which binaries; and there is also a script
-called \cw{mkfiles.pl}, which reads \c{Recipe} and writes out the
-real \cw{Makefile}s. (The script also reads all the source files and
-analyses their dependencies on header files, so we get an extra
-benefit from doing it this way, which is that we can supply correct
-dependency information even in environments where it's difficult to
-set up an automated \c{make depend} phase.)
-
-You should \e{never} edit any of the PuTTY \cw{Makefile}s directly.
-They are not stored in our source repository at all. They are
-automatically generated by \cw{mkfiles.pl} from the file \c{Recipe}.
-
-If you need to add a new object file to a particular binary, the
-right thing to do is to edit \c{Recipe} and re-run \cw{mkfiles.pl}.
-This will cause the new object file to be added in every tool that
-requires it, on every platform where it matters, in every
-\cw{Makefile} to which it is relevant, \e{and} to get all the
-dependency data right.
-
-If you send us a patch that modifies one of the \cw{Makefile}s, you
-just waste our time, because we will have to convert it into a
-change to \c{Recipe}. If you send us a patch that modifies \e{all}
-of the \cw{Makefile}s, you will have wasted a lot of \e{your} time
-as well!
-
-(There is a comment at the top of every \cw{Makefile} in the PuTTY
-source archive saying this, but many people don't seem to read it,
-so it's worth repeating here.)
-
 \H{udp-ssh-coroutines} Coroutines in the SSH code
 
 Large parts of the code in the various SSH modules (in fact most of