- Fix ccache for CURRENT world builds by automatically defining
CCACHE_CPP2=1 when COMPILER_TYPE=clang (this will be removed
once upstream fixes the issue). This does not address port
builds yet.
- Add more hints for building with clang
- Split MASTER_SITES into 2 lines
- Note that textproc/libexttextcat is an example port that currently
fails to build with clang+ccache due to bug 8118 ('unused -I'). I
plan to add the upstream patch to fix this after more testing.