WITH_SYSTEM_COMPILER: Disable with WITH_META_MODE for now.

WITH_SYSTEM_COMPILER: Disable with WITH_META_MODE for now.

The WITH_META_MODE build is intended to be a working incremental build.
It spies on the build command to see if things should be rebuilt if the
command changes.  If you run buildworld, it builds a cross-compiler,
then do installworld and buildworld again it will invoke the
WITH_SYSTEM_COMPILER logic.  This then adds on -target/--sysroot, etc,
and causes rebuilds due to the changed build command even though the
compiler used is technically the same revision.  Since the incremental
build is not cleaning anything by default then there is much
less risk to rebuilding the already-existing cross-compiler.  Just
disable the combined logic and always use and build the cross-compiler.

An alternative to this would be to always pass -target/--sysroot.  Doing
so may occur in the future.

Sponsored by:	EMC / Isilon Storage Division