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.

1 files changed, 3 insertions, 0 deletions

diff --git a/CHECKLST.txt b/CHECKLST.txt
index c3a5365a..82fc9d6a 100644
--- a/CHECKLST.txt
+++ b/CHECKLST.txt
@@ -37,6 +37,9 @@ Things to do during the branch-stabilisation period:
    particular, any headline features for the release should get a
    workout with memory checking enabled!
 
+ - Test the CMake build scripts with the oldest CMake they claim to
+   support, on both Unix and Windows.
+
 Making a release candidate build
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