xfsprogs: LDFLAGS comes from configure, not environment

xfsprogs: LDFLAGS comes from configure, not environment

When doing:

$ LDFLAGS=foo make

bad things happen because we don't initialise LDFLAGS to an empty
string in include/builddefs.in and hence make takes wahtever is in
the environment and runs with it. This causes problems with linker
options specified correctly through configure.

We don't support overriding build flags (like CFLAGS) though the
make environment, so it was an oversight 13 years ago to allow
LDFLAGS to be overridden when adding support to custom LDFLAGS being
passed from the the configure script. This ensures we only ever use
linker flags from configure, not the make environment.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>