Update configure check to require GMP-6.0.0 or later.

ChangeLog

 2018-11-28  Niels Möller  <nisse@lysator.liu.se>
 
+	* configure.ac: Update GMP check. Check for the function
+	mpn_sec_div_r, available since GMP-6.0.0.
+
 	* testsuite/rsa-encrypt-test.c (test_main): Fix allocation of
 	decrypted storage. Update test of rsa_decrypt, to allow clobbering
 	of all of the passed in message area.

configure.ac

@@ -236,9 +236,9 @@ fi
 # Checks for libraries
 if test "x$enable_public_key" = "xyes" ; then
   if test "x$enable_mini_gmp" = "xno" ; then
-    AC_CHECK_LIB(gmp, __gmpz_powm_sec,,
+    AC_CHECK_LIB(gmp, __gmpn_sec_div_r,,
         [AC_MSG_WARN(
-    [GNU MP not found, or too old. GMP-5.0 or later is needed, see http://gmplib.org/.
+    [GNU MP not found, or too old. GMP-6.0 or later is needed, see https://gmplib.org/.
     Support for public key algorithms will be unavailable.])]
         enable_public_key=no)