meson: stop Meson using target CFLAGS in native builds
With the goal of autoconf-compatibility Meson respects $CFLAGS et al in builds.
In cross-compiled build the cross file is the one true source of flags and the
environment isn't used, but in a native build the environment will still be
respected.
As this can lead to target flags being used in the build for native binaries
(including a single native binary inside a target recipe), export
CFLAGS=${BUILD_CFLAGS) et al.
Signed-off-by: Ross Burton <ross.burton@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
