Revert r256921 to prevent error output when in the wrong directory.
This should have been reverted with the stable/10/Makefile.inc1
revert, but apparently my commit did not go through.

Discussed with:	cperciva (originally)
Regenerate documentation post r256915:

Stop conflating WITHOUT_CLANG with WITHOUT_CLANG_IS_CC.  This allows
bootstrapping a copy of clang without building clang for the base system
which is useful for nanobsd and similar setups.  It's still probably
wrong to conflate what is installed as /usr/bin/cc with the selection
of a bootstrap compiler under WITH*_CLANG_IS_CC, but that's for another
day.