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.

cmake/setup.cmake

@@ -55,9 +55,19 @@ add_custom_target(cmake_commit_c
   DEPENDS check_git_commit ${INTERMEDIATE_COMMIT_C}
   COMMENT "Updating cmake_commit.c")
 
+if(CMAKE_VERSION VERSION_LESS 3.12)
+  function(add_compile_definitions)
+    foreach(i ${ARGN})
+      add_compile_options(-D${i})
+    endforeach()
+  endfunction()
+endif()
+
 function(add_sources_from_current_dir target)
-  set(sources ${ARGN})
-  list(TRANSFORM sources PREPEND ${CMAKE_CURRENT_SOURCE_DIR}/)
+  set(sources)
+  foreach(i ${ARGN})
+    set(sources ${sources} ${CMAKE_CURRENT_SOURCE_DIR}/${i})
+  endforeach()
   target_sources(${target} PRIVATE ${sources})
 endfunction()