- Allow WITH_CCACHE_BUILD to work with boost.
This speeds up building from 35 minutes to 5 minutes.
If you are building boost with ccache by changing CC="ccache cc",
change to using WITH_CCACHE_BUILD=yes in make.conf. This will
automatically handle installing and using ccache for port building,
while avoiding build breakages.
Reported by: Samu Nuutamo
Discussed with: bapt (who marked it unsafe)