Tests/CompileDefinitions: Avoid spaces in defines on VS 6

The VS 6 IDE does not like spaces in definition values so CMake drops
them and warns.  The Tests/CompileDefinitions test C code that looks for
the dropped definitions already knows to skip them, but CMake still
warns.  Silence the warnings by avoiding such values in the first place
on VS 6.
Merge topic 'silence-vs6-warnings' into next

015309f Tests/CompileDefinitions: Avoid spaces in defines on VS 6