CMakeLists.txt: explicitly ask for C99.

A user just reported that 0.79 doesn't build out of the box on Ubuntu
14.04 (trusty), because although its gcc (4.8.4) does _support_ C99,
it doesn't enable it without a non-default -std option. The user was
able to work around the problem by defining CMAKE_C_FLAGS=-std=gnu99,
but it would have been nicer if we'd done that automatically. Setting
CMAKE_C_STANDARD causes cmake to do so.

(This isn't a regression of 0.79 over 0.78 as far as I know; the user
in question said they had last built 0.76.)

I was surprised to find Ubuntu 14.04 still in use at all, but a quick
web search revealed that its support has been extended until next
year, so fair enough, I suppose. It's also running a cmake way older
than we support, but apparently that can be worked around via
Kitware's binary tarball downloads (which do still run on 14.04).

This is a bit unsatisfactory: I'd prefer to ask for C standards
support of _at least_ C99 level, and C11 if possible. Then I could
test for the presence of C11 features via check_c_source_compiles, and
use them opportunistically (e.g. in macro definitions). But as far as
I can see, cmake has no built-in support for asking for a standards
level of 'as new as you can get, but no older than 99'. Oh well.

(In any case, the thing I'd find most useful from C11 is _Generic, and
since that's in implementation namespace, compilers can - and do -
support it in C99 mode anyway. So it's probably fine, at least for now.)

CMakeLists.txt

@@ -1,6 +1,8 @@
 cmake_minimum_required(VERSION 3.7)
 project(putty LANGUAGES C)
 
+set(CMAKE_C_STANDARD 99)
+
 include(cmake/setup.cmake)
 
 # Scan the docs directory first, so that when we start calling