Don't hash compiler option -fdebug-prefix-map=

The -fdebug-prefix-map option may be used in combination with
CCACHE_BASEDIR to reuse results across different directories.

Based on a patch by Pawel Sikora.
doc: Mention that --ccache-skip currently does not mean "skip hashing option"
Require CCACHE_SLOPPINESS=pch_defines,time_macros opt-in to enable PCH handling

The background is that since ccache runs the preprocessor on the header to
be precompiled, the preprocessor removes #defines and ccache then hashes
the output, which means that changes in #defines are not detected even
though the resulting .gch file will have different behavior.