Make PGAC_C_BUILTIN_OP_OVERFLOW link instead of just compiling.

Otherwise the detection can spuriously detect symbol as available,
because the compiler may just emits reference to non-existant symbol.

config/c-compiler.m4

@@ -305,10 +305,11 @@ 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.
+# proxy for all of the operations. Have to link to be sure to
+# recognize a missing __builtin_mul_overflow.
 AC_DEFUN([PGAC_C_BUILTIN_OP_OVERFLOW],
 [AC_CACHE_CHECK(for __builtin_mul_overflow, pgac_cv__builtin_op_overflow,
-[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],
+[AC_LINK_IFELSE([AC_LANG_PROGRAM([],
 [PG_INT64_TYPE result;
 __builtin_mul_overflow((PG_INT64_TYPE) 1, (PG_INT64_TYPE) 2, &result);]
 )],

configure

@@ -14472,6 +14472,8 @@ esac
 
 fi
 
+# has to be down here, rather than with the other builtins, because
+# the test uses PG_INT64_TYPE.
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for __builtin_mul_overflow" >&5
 $as_echo_n "checking for __builtin_mul_overflow... " >&6; }
 if ${pgac_cv__builtin_op_overflow+:} false; then :
@@ -14490,12 +14492,13 @@ __builtin_mul_overflow((PG_INT64_TYPE) 1, (PG_INT64_TYPE) 2, &result);
   return 0;
 }
 _ACEOF
-if ac_fn_c_try_compile "$LINENO"; then :
+if ac_fn_c_try_link "$LINENO"; then :
   pgac_cv__builtin_op_overflow=yes
 else
   pgac_cv__builtin_op_overflow=no
 fi
-rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$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; }