Fix checking for existing builtin overflow functions

This patch fixes commit 1ca5486, which added
support of using builtin overflow functions: __builtin_add_overflow,
__builtin_sub_overflow, __builtin_mul_overflow. The configure script tries to
compile a small program with one of these functions. Compilation was without any
errors, because to correctly check gcc requires flag
-Werror=implicit-function-declaration. This flag is added later.

This patch moves this check after flag -Werror=implicit-function-declaration is
added to CFLAGS.

configure

@@ -17614,39 +17614,6 @@ esac
 
 fi
 
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for __builtin_mul_overflow" >&5
-$as_echo_n "checking for __builtin_mul_overflow... " >&6; }
-if ${pgac_cv__builtin_op_overflow+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
-/* end confdefs.h.  */
-
-int
-main ()
-{
-PG_INT64_TYPE result;
-__builtin_mul_overflow((PG_INT64_TYPE) 1, (PG_INT64_TYPE) 2, &result);
-
-  ;
-  return 0;
-}
-_ACEOF
-if ac_fn_c_try_compile "$LINENO"; then :
-  pgac_cv__builtin_op_overflow=yes
-else
-  pgac_cv__builtin_op_overflow=no
-fi
-rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
-fi
-{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv__builtin_op_overflow" >&5
-$as_echo "$pgac_cv__builtin_op_overflow" >&6; }
-if test x"$pgac_cv__builtin_op_overflow" = xyes ; then
-
-$as_echo "#define HAVE__BUILTIN_OP_OVERFLOW 1" >>confdefs.h
-
-fi
-
 # Check size of void *, size_t (enables tweaks for > 32bit address space)
 # The cast to long int works around a bug in the HP C Compiler
 # version HP92453-01 B.11.11.23709.GP, which incorrectly rejects
@@ -19823,6 +19790,39 @@ fi
 
 fi
 
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for __builtin_mul_overflow" >&5
+$as_echo_n "checking for __builtin_mul_overflow... " >&6; }
+if ${pgac_cv__builtin_op_overflow+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+PG_INT64_TYPE result;
+__builtin_mul_overflow((PG_INT64_TYPE) 1, (PG_INT64_TYPE) 2, &result);
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  pgac_cv__builtin_op_overflow=yes
+else
+  pgac_cv__builtin_op_overflow=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv__builtin_op_overflow" >&5
+$as_echo "$pgac_cv__builtin_op_overflow" >&6; }
+if test x"$pgac_cv__builtin_op_overflow" = xyes ; then
+
+$as_echo "#define HAVE__BUILTIN_OP_OVERFLOW 1" >>confdefs.h
+
+fi
+
 # Begin output steps
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: using CFLAGS=$CFLAGS" >&5