When building g++, we need to build libsupc earlier to avoid a race
with libproc. Not sure why people didn't see this before now, but I
get it often for higher (20-30) -j builds, but never for -j1 builds.
Allow the build system to safely set MK_FOO to avoid the ambiguity
when both WITH_FOO and WITHOUT_FOO are set. Use this where
possible. Only disallow setting of MK_FOO on the command line. This
was preferable to inventing a new mechanism or fixing the undef bug
(bin/183762) which precludes users from turning off anything we turn
off for parts of the build with WITHOUT_FOO prior to this.
Kill NO_TESTS and use MK_TESTS=no when we want to skip them.
Move stray targets out of the xdev section.