WITH_SYSTEM_COMPILER default on

This feature is where the bootstrap compiler in buildworld is not built
if the one in /usr/bin/cc matches what would be built.  It is very
conservative and requires a complete match of major/minor version and
the compiler revision (__FreeBSD_cc_version).

The only complaint so far about this feature was that when we bumped the
compiler revision, too much of clang would rebuild.  That was addressed
in r301277.

I would like to enable this feature by default in head for 11.0.

Unless there are any objections I will do so in a few days.

-- 
Regards,
Bryan Drewery