xfsprogs: Add BUILD_CFLAGS for cross compilation

[Christoph Hellwig]

On Mon, Feb 01, 2016 at 01:25:09PM -0800, Gwendal Grignou wrote:
> When cross compiling, CFLAGS may contains option for the cros-compiler the
> host compiler can not handle.
> Add BUILD_CFLAGS variable that does not include $OPTIMIZER options.

I would have expected this to be HOST_CFLAGS, but it seems the
BUILD_ convention already exists in xfsprogs, so sticking to it
looks fine to me.

Reviewed-by: Christoph Hellwig <hch at lst.de>