execinfo: port to FreeBSD * m4/execinfo.m4 (gl_EXECINFO_H): Set LIB_EXECINFO to -lexecinfo if needed, as in FreeBSD. Reported by Bastien Roucariès in <http://lists.gnu.org/archive/html/bug-gnulib/2012-08/msg00113.html>. * modules/execinfo (Link): Add $(LIB_EXECINFO).