build: Allow compiling xfsprogs in a cross compile environment

build: Allow compiling xfsprogs in a cross compile environment

Without this patch, we are using the same compiler and options for the host
compiler (BUILD_CC) and the target compiler (CC), and we would get error
messages at compilation:
x86_64-pc-linux-gnu-gcc -O2 -O2 -pipe -march=armv7-a -mtune=cortex-a15 ...
x86_64-pc-linux-gnu-gcc.real: error: unrecognized command line option
'-mfpu=neon'
'-mfloat-abi=hard'
'-clang-syntax'
'-mfpu=neon'
'-mfloat-abi=hard'
'-clang-syntax'

Add BUILD_CC and BUILD_CFLAGS as precious variables to allow setting it up
from the ebuild.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Mike Frysinger <vapier@gentoo.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>