gcc-5.2: Fix various _FOR_BUILD and related variables
When doing a FOR_BUILD thing, you have to override CFLAGS with
CFLAGS_FOR_BUILD. And if you use C++, you also have to override
CXXFLAGS with CXXFLAGS_FOR_BUILD.
Without this, when building for mingw, you end up trying to use
the mingw headers for a host build.
The same goes for other variables as well, such as CPPFLAGS,
CPP, and GMPINC.
Signed-off-by: Peter Seebach <peter.seebach@windriver.com> Signed-off-by: Mark Hatle <mark.hatle@windriver.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
