autogen: Run parallel make if compiling with distcc.

autogen: Run parallel make if compiling with distcc.

This adjusts the value of the make -j option in case the DISTCC_HOSTS
environment variable is set. There is no way to determine the number of
processing units (cores) for the remote hosts listed in DISTCC_HOSTS,
so we set the number of jobs to run simultaneously to

	n + 2 * n * d

where n is the number of cores on the local system and d is the
number of hosts in DISTCC_HOSTS. This seems to work fine on one such
distributed setup.