Try harder to detect unavailability of __builtin_mul_overflow(int64).

Commit c04d35f4 didn't quite do the job here, because it still allowed
the compiler to deduce that the function call could be optimized away.
Prevent that by putting the arguments and results in global variables.

Discussion: https://postgr.es/m/20171213213754.pydkyjs6bt2hvsdb@alap3.anarazel.de

config/c-compiler.m4

@@ -310,18 +310,19 @@ fi])# PGAC_C_BUILTIN_CONSTANT_P
 # and define HAVE__BUILTIN_OP_OVERFLOW if so.
 #
 # Check for the most complicated case, 64 bit multiplication, as a
-# proxy for all of the operations. Use volatile variables to avoid the
-# compiler computing result at compile time, even though the runtime
-# might not supply operation. Have to link to be sure to recognize a
-# missing __builtin_mul_overflow.
+# proxy for all of the operations.  To detect the case where the compiler
+# knows the function but library support is missing, we must link not just
+# compile, and store the results in global variables so the compiler doesn't
+# optimize away the call.
 AC_DEFUN([PGAC_C_BUILTIN_OP_OVERFLOW],
 [AC_CACHE_CHECK(for __builtin_mul_overflow, pgac_cv__builtin_op_overflow,
-[AC_LINK_IFELSE([AC_LANG_PROGRAM([],
-[PG_INT64_TYPE a = 1;
+[AC_LINK_IFELSE([AC_LANG_PROGRAM([
+PG_INT64_TYPE a = 1;
 PG_INT64_TYPE b = 1;
 PG_INT64_TYPE result;
-__builtin_mul_overflow(*(volatile PG_INT64_TYPE *) a, *(volatile PG_INT64_TYPE *) b, &result);]
-)],
+int oflo;
+],
+[oflo = __builtin_mul_overflow(a, b, &result);])],
 [pgac_cv__builtin_op_overflow=yes],
 [pgac_cv__builtin_op_overflow=no])])
 if test x"$pgac_cv__builtin_op_overflow" = xyes ; then

configure

@@ -14485,14 +14485,15 @@ else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 
-int
-main ()
-{
 PG_INT64_TYPE a = 1;
 PG_INT64_TYPE b = 1;
 PG_INT64_TYPE result;
-__builtin_mul_overflow(*(volatile PG_INT64_TYPE *) a, *(volatile PG_INT64_TYPE *) b, &result);
+int oflo;
 
+int
+main ()
+{
+oflo = __builtin_mul_overflow(a, b, &result);
   ;
   return 0;
 }