automake: new option posix to emit .POSIX target first. This patch is from https://bugs.gnu.org/55025. * lib/Automake/Options.pm (_is_valid_easy_option): declare new option "posix". * bin/automake.in (generate_makefile): if the "posix" option is present, add a .POSIX special target as the first non-comment line in the Makefile.in files. * NEWS: mention this. * doc/automake.texi: likewise.