configure: Pass CPPFLAGS_FOR_BUILD to subdirs

configure: Pass CPPFLAGS_FOR_BUILD to subdirs

Because CPPFLAGS_FOR_BUILD is used in some subdirectories (through
bfd/warning.m4), not AC_SUBSTing the variable causes minor issues.

Fortunately, it didn't cause severe errors but error messages related to
@CPPFLAGS_FOR_BUILD@ (not AC_SUBSTed CPPFLAGS_FOR_BUILD variable passed
to subdirectories through Makefile) remain in config.log.

To avoid invalid invocation of preprocessor for build environment, we
need to set proper CPPFLAGS_FOR_BUILD (may be empty) and pass it to
subdirectories that need it.  This is what this commit does.

ChangeLog:

	* configure.ac: Pass CPPFLAGS_FOR_BUILD to subdirectories.
	* configure: Regenerate.