gcc: introduce --posixly command line option to ease the compilation of POSIX software

cross/patch/gcc/gcc/config/jehanne.opt

@@ -0,0 +1,25 @@
+; Jehanne options.
+
+; This file is part of Jehanne.
+;
+; Copyright (C) 2016-2020 Giacomo Tesio <giacomo@tesio.it>
+;
+; Jehanne is free software: you can redistribute it and/or modify
+; it under the terms of the GNU General Public License as published by
+; the Free Software Foundation, version 2 of the License.
+;
+; Jehanne is distributed in the hope that it will be useful,
+; but WITHOUT ANY WARRANTY; without even the implied warranty of
+; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+; GNU General Public License for more details.
+;
+; You should have received a copy of the GNU General Public License
+; along with Jehanne.  If not, see <http://www.gnu.org/licenses/>.
+
+; Please try to keep this file in ASCII collating order.
+
+-posixly
+C ObjC C++ ObjC++ Driver RejectNegative
+Lookup for POSIX includes and libraries in /posix
+
+; This comment is to ensure we retain the blank line above.