pkg.m4 is licensed under GPL-2 so LICENSE field should reflect that, the same way dev-util/pkgconfig-openbsd is doing. In this case: LICENSE="BSD-1 pkg-config? ( GPL-2 )"
oups, I've totally missed the exception: # As a special exception to the GNU General Public License, if you # distribute this file as part of a program that contains a # configuration script generated by Autoconf, you may include it under # the same distribution terms that you use for the rest of that program. so just ignore me