fixing vs WARN_CFLAGS vs cross builds

[DJ Delorie]

For background, I'm doing this:

	--build=i686-pc-linux-gnu
	--host=i586-pc-msdosdjgpp
	--target=i586-pc-msdosdjgpp

So, it's a native build, but cross-built.  DJGPP *does* use
fixincludes, so that should run.

Apparently, configure tests $CC for support of -Wno-long-long
-Wno-variadic-macros (and a few others) and adds them to the stage1
flags if supported.  However, $CC is a cross compiler, different
from $BUILD_CC.

"If the native compiler is GCC, we can enable warnings even in stage1."

Well, it is GCC, and strict1 propogates to WARN_CFLAGS, and
WARN_CFLAGS is passed to fixinc.  But the compiler used in fixinc is a
different gcc (BUILD_CC vs CC).