gcc-5.2: Fix various _FOR_BUILD and related variables

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.

(From OE-Core rev: 85ca40c42950315f2783b98f57df16b261d2826e)

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