META MODE: Fix MACHINE=host builds.

We need to not use -nostdinc since it breaks building of clang itself. Use
-isystem rather than -I/usr/include and -nostdinc which gets us using
the stage include directory before searching the real host headers.
This allows removing more of the -I hacks to get host headers since the
headers are no longer excluded. The -B seemed unneeded.

This fixes building of secure/lib/libcrypto which was looking at the
/usr/include/openssl/asn1.h header rather than the staged one.

This fixes building of clang which wants to find its own internal
headers in the STAGEDIR/usr/lib/clang/* path.

Sponsored by: EMC / Isilon Storage Division