Compatibility with older versions of cmake.

After this change, the cmake setup now works even on Debian stretch
(oldoldstable), which runs cmake 3.7.

In order to support a version that early I had to:

 - write a fallback implementation of 'add_compile_definitions' for
   older cmakes, which is easy, because add_compile_definitions(FOO)
   is basically just add_compile_options(-DFOO)

 - stop using list(TRANSFORM) and string(JOIN), of which I had one
   case each, and they were easily replaced with simple foreach loops

 - stop putting OBJECT libraries in the target_link_libraries command
   for executable targets, in favour of adding $<TARGET_OBJECTS:foo>
   to the main sources list for the same target. That matches what I
   do with library targets, so it's probably more sensible anyway.

I tried going back by another Debian release and getting this cmake
setup to work on jessie, but that runs CMake 3.0.1, and in _that_
version of cmake the target_sources command is missing, and I didn't
find any alternative way to add extra sources to a target after having
first declared it. Reorganising to cope with _that_ omission would be
too much upheaval without a very good reason.

doc/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.7)
 project(putty-documentation LANGUAGES)
 
 # This build script can be run standalone, or included as a