xfstests: Automatic build dependency calculations

xfstests: Automatic build dependency calculations

Currently the xfstest builds do not have any automatic dependency
calculations. It relies on a separate make depend run to build or
update dependency information. It also relies on an external
makedepend binary. If that binary does not exist, the dependencies
do not get calculated.

To remove the dependency on makedepend, gcc can be used instead as
it has a command to generate dependency information. This patch
changes the dependency rule building to use gcc.

In case anyone uses an old (several years) gcc compiler or a
compiler that doesn't support gcc compatible dependency generation,
a new configure check is added to turn off dependency checking so
builds can still be done.

To use the dependencies automatically, we need to use a special
include makefile directive to include the build dependencies into
the current makefile.  Essentially once the dependencies are
calculated, they can be included into the makefile and make will
recalculate the build dependencies automatically based on that
information.

Hence we get a build that automatically calculates and keeps
dependencies up to date without dependence on any external tools.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>