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/mpf/set_d.c b/third_party/gmp/mpf/set_d.c
new file mode 100644
index 0000000..0442f2f
--- /dev/null
+++ b/third_party/gmp/mpf/set_d.c
@@ -0,0 +1,59 @@
+/* mpf_set_d -- Assign a float from a double.
+
+Copyright 1993-1996, 2001, 2003, 2004 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/.  */
+
+#include "config.h"
+
+#if HAVE_FLOAT_H
+#include <float.h>  /* for DBL_MAX */
+#endif
+
+#include "gmp-impl.h"
+
+void
+mpf_set_d (mpf_ptr r, double d)
+{
+  int negative;
+
+  DOUBLE_NAN_INF_ACTION (d,
+                         __gmp_invalid_operation (),
+                         __gmp_invalid_operation ());
+
+  if (UNLIKELY (d == 0))
+    {
+      SIZ(r) = 0;
+      EXP(r) = 0;
+      return;
+    }
+  negative = d < 0;
+  d = ABS (d);
+
+  SIZ(r) = negative ? -LIMBS_PER_DOUBLE : LIMBS_PER_DOUBLE;
+  EXP(r) = __gmp_extract_double (PTR(r), d);
+}