--- a/libquadmath/acinclude.m4	Sun Aug 21 07:07:55 2011 +0900
+++ b/libquadmath/acinclude.m4	Fri Oct 27 22:46:09 2017 +0900
@@ -10,3 +10,20 @@
 AC_DEFUN([AC_LIBTOOL_DLOPEN])
 AC_DEFUN([AC_PROG_LD])
 ])
+
+dnl Check whether POSIX's signgam is defined in math.h.
+AC_DEFUN([LIBQUAD_CHECK_MATH_H_SIGNGAM], [
+  AC_CACHE_CHECK([whether the math.h includes POSIX's signgam],
+                 libgfor_cv_have_math_h_signgam, [
+  save_CFLAGS="$CFLAGS"
+  CFLAGS="$CFLAGS -Werror"
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+#include <math.h>
+void foo(void) { signgam = 1; }]], [])],
+                    libgfor_cv_have_math_h_signgam=yes,
+                    libgfor_cv_have_math_h_signgam=no)
+  CFLAGS="$save_CFLAGS"])
+  if test $libgfor_cv_have_math_h_signgam = yes; then
+    AC_DEFINE(HAVE_MATH_H_SIGNGAM, 1,
+      [Define to 1 if the math.h includes POSIX's signgam.])
+  fi])