Always default CCACHE_CPP2 to 1, otherwise a build error with parenthesis can
occur in buildworld when upgrading from a GCC-built clang or pre-10 system to a
system built with clang.
Fix use with clang. CCACHE_CPP2 is still required. Otherwise the
errors with parenthesis occur again.
Seen with www/uwsgi.
Reported by: blackflow on freenode