pkg-autotools: use --disable-dependency-tracking

By default, automake does "dependency tracking", which allows the
generated Makefile to contain the necessary dependencies to
automatically rebuild the appropriate C files when included header
files are changed. This dependency tracking is nice when doing active
development on the package, but not really useful when doing a
one-time build of the package. According to automake's
documentation[1], disabling the dependency tracking provides a small
speed-up.

In some very unscientific measurements (i.e repeated only once), we
have noticed a ~3.6% reduction of the total build time of a Buildroot
toolchain after applying this patch.

[1] http://www.gnu.org/software/automake/manual/html_node/Dependency-Tracking.html



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>