CCache requires this flag when building with precompiled headers.
Without it, the preprocessor fails and CCache fallbacks to running the
real compiler.
Users still need to set 'sloppiness' to 'pch_defines,time_macros' in
their ccache.conf file for CCache to cache builds that use precompiled
headers. See the CCache manual for more info:
https://ccache.samba.org/manual.html#_precompiled_headers