Add libgmp 6.2.0 to third_party

Don't build it yet.  That will come in the next review.

Change-Id: Idf3266558165e5ab45f4a41c98cc8c838c8244d5
diff --git a/third_party/gmp/doc/Makefile.am b/third_party/gmp/doc/Makefile.am
new file mode 100644
index 0000000..083f25a
--- /dev/null
+++ b/third_party/gmp/doc/Makefile.am
@@ -0,0 +1,36 @@
+## Process this file with automake to generate Makefile.in
+
+
+# Copyright 2003 Free Software Foundation, Inc.
+#
+#  This file is part of the GNU MP Library.
+#
+#  The GNU MP Library is free software; you can redistribute it and/or modify
+#  it under the terms of either:
+#
+#    * the GNU Lesser General Public License as published by the Free
+#      Software Foundation; either version 3 of the License, or (at your
+#      option) any later version.
+#
+#  or
+#
+#    * the GNU General Public License as published by the Free Software
+#      Foundation; either version 2 of the License, or (at your option) any
+#      later version.
+#
+#  or both in parallel, as here.
+#
+#  The GNU MP Library 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 copies of the GNU General Public License and the
+#  GNU Lesser General Public License along with the GNU MP Library.  If not,
+#  see https://www.gnu.org/licenses/.
+
+
+EXTRA_DIST = configuration isa_abi_headache projects.html tasks.html
+
+info_TEXINFOS = gmp.texi
+gmp_TEXINFOS = fdl-1.3.texi