Improve ccache implementation

Improve ccache implementation

Renamed the launch-c* scripts to make clear that they are supporting
infrastructure for ccache, so maintainers know what the script is for
without having to read it.

Moved them into the admin directory, which contains other
development-time resources. This also means that these scripts are no
longer in the top-level directory of the unpacked tarball, where they
might confuse users. ccache will still work for builds from the
tarball, however.

Eliminated temporary helper variables that didn't add value to
how we use configure_file().

Renamed GMX_CACHE_.*_COMPILER variables to use CCACHE rather
than CACHE, for consistency.

This change would have broken build trees that used to work with
ccache, because the wrapper scripts may not be regenerated
because the old GMX_CACHE_* variables are defined. But
changing to GMX_CCACHE_* has the side effect of fixing this
because the new variables will be undefined in such build trees.

Change-Id: I3b449251ee06adc8023ac1a58cb98a7b0be9bba5