Detect --disable-dependency-tracking in Makefile.am

[Dave Hart]

I've added code to the ntp.org Makefile.am files to ensure the static
utility library libntp.a is up-to-date for each program that uses it, to
ensure the build is correct.  When building the project, this adds a bunch
of extra submake invocations which slows down the build.  I'd like to omit
those when --disable-dependency-tracking is used, to speed one-off builds
that are done by end-users and packagers.

I'm guessing someone has trod this ground before.  I'd appreciate pointers
to examples of how others have detected --disable-dependency-tracking to
change their build behavior.

Thanks in advance.
Dave Hart