During cross compiling, CPP is being set to the target compiler even for
build targets. As an example, when building a cross compiler targetting
mingw, the config.log for libiberty in
build.x86_64-pokysdk-mingw32.i586-poky-linux/build-x86_64-linux/libiberty/config.log
shows:

configure:3786: checking how to run the C preprocessor
configure:3856: result: x86_64-pokysdk-mingw32-gcc -E --sysroot=[sysroot]/x86_64-nativesdk-mingw32-pokysdk-mingw32
configure:3876: x86_64-pokysdk-mingw32-gcc -E --sysroot=[sysroot]/x86_64-nativesdk-mingw32-pokysdk-mingw32 conftest.c
configure:3876: $? = 0

This is libiberty being built for the build environment, not the target one
(i.e. in build-x86_64-linux). As such it should be using the build environment's
gcc and not the target one. In the mingw case the system headers are quite
different leading to build failures related to not being able to include a
process.h file for pem-unix.c.

Further analysis shows the same issue occuring for CPPFLAGS too.

Fix this by adding support for CPP_FOR_BUILD and CPPFLAGS_FOR_BUILD which
for example, avoids mixing the mingw headers for host binaries on linux
systems.

2021-10-27 Richard Purdie <richard.purdie@linuxfoundation.org>

ChangeLog:

	* Makefile.tpl: Add CPP_FOR_BUILD and CPPFLAGS_FOR_BUILD support
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Add CPP_FOR_BUILD and CPPFLAGS_FOR_BUILD support

gcc/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Use CPPFLAGS_FOR_BUILD for GMPINC

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>