Regenerate with automake-1.15.1

Until we update the recommended versions of autoconf/automake, files
should be regenerated with automake-1.15.1 and autoconf-2.69.  That's
not because we think those versions are golden, and newer versions are
bad.  It's simply because maintainers want to be able to update
configury files without trouble, and if someone regenerates files with
automake-1.16.5 then --enable-maintainer-mode builds will hit errors:

checking that generated files are newer than configure... configure.ac:26: error: version mismatch.  This is Automake 1.15.1,
configure.ac:26: but the definition used by this AM_INIT_AUTOMAKE
configure.ac:26: comes from Automake 1.16.5.  You should recreate
configure.ac:26: aclocal.m4 with aclocal and run automake again.
WARNING: 'automake-1.15' is probably too old.

Correcting this requires regenerating the files by hand.