Fix building xfsprogs on 32-bit platforms

Fix building xfsprogs on 32-bit platforms

xfslibs now requires that its users enable transparent largefile
support.  This broke building xfsprogs on 32-bit Linux (with glibc)
because _FILE_OFFSET_BITS=64 was not getting defined.  Although the
autoconf macro AC_SYS_LARGEFILE was intended to define it, this didn't
work because AC_SYS_LARGEFILE will only define _FILE_OFFSET_BITS in a
config header, which doesn't work for xfsprogs because not all .c files
include platform_defs.h as their first include.  Also,
platform_defs.h.in is not generated by autoheader and didn't contain a
template for _FILE_OFFSET_BITS.

Therefore, to fix the problem remove the useless autoconf macros and
instead add -D_FILE_OFFSET_BITS=64 to CFLAGS in builddefs.in.  Use
CFLAGS rather than PCFLAGS because this definition could be needed by
platforms other than "linux", and it doesn't hurt to always define it.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Felix Janda <felix.janda@posteo.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>