The check activated ccache unconditionally for all non-Clang compilers
(= GCC) while allowing ancient ccache versions for these. Perhaps there
was a reason for that in the past but it's simpler to only require a
minimum ccache version.
To simplify further, also require at least ccache 3.3 (released in 2016)
instead of 3.2.1 (released in 2014).
The compiler launcher (ccache, distcc) can be set externally via
-DCMAKE_CXX_COMPILER_LAUNCHER=<tool>. We previously silently ignored
this setting and continued without any launcher (e.g. ccache). Changed
this to now respect the externally specified launcher.
