xfsprogs: fix build dep on configure.ac

xfsprogs: fix build dep on configure.ac

Zorro reported that this sequence:

# git checkout v4.9.0; make realclean; make
# git checkout v4.10.0; make clean; make

fails:

...
Building libxfs
    [CC]     gen_crc32table
gcc: error: @BUILD_CFLAGS@: No such file or directory
gmake[3]: *** No rule to make target `crc32table.h', needed by `crc32selftest'.  Stop.

This is because

0a71e38 build: Allow compiling xfsprogs in a cross compile environment

added the new BUILD_CFLAGS to configure.ac, and unless we re-run
autotools, that variable does not get substituted when
include/builddefs gets built.

(This can be worked around by "make realclean" and then everything
gets regenerated.)

The configure script is generated from configure.ac, so adding
a Make dependency here should resolve such issues in the future.

Reported-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>