hg: build-infra/jdk9: Make gcc 4.8 detetion hack overridable from custom extensions.

[magnus.ihse.bursie at oracle.com]

Changeset: e18d76153658
Author:    ihse
Date:      2016-02-04 22:01 +0100
URL:       http://hg.openjdk.java.net/build-infra/jdk9/rev/e18d76153658

Make gcc 4.8 detetion hack overridable from custom extensions.

! common/autoconf/flags.m4
! common/autoconf/generated-configure.sh