PORTS_MODULES: Don't leak in CC/CXX/CPP.

These may have ccache in them or -target/--sysroot from external
compiler or SYSTEM_COMPILER support. Many ports do not support
a CC with spaces in it, such as emulators/virtualbox-ose-kmod.
Passing --sysroot to ports makes no sense as ports doesn't support
--sysroot currently.
If these variables need to be overridden for ports then they can
be set in make.conf or passed as make arguments.

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division