As the final (for now) cleanup of autotools, in order to reduce tree
churn on new versions, a new target has been introduced to allow
PLIST_SUB-style patching of files. For example, rather than patching
"foo/bar/baz" to tweak some random occurance of an autoconf version number
to the latest, the patch should now replace it with %%AUTOCONF_VERSION%%

A new Makefile variable, AUTOTOOLSFILES has been introduced, which when
populated will automatically go through and patch %%AUTOCONF_VERSION%% to
the contents of ${AUTOCONF_VERSION} (and likewise for all other known
autotools variables -- "make -V AUTOTOOLS_VARS" to see the list for
a specific port.

Tested by: pav (-exp run, since it touches bsd.port.mk also)