hg: build-infra/jdk8: If ALT_ variables are set, just warn about it, do not exit. But warn both at

[magnus.ihse.bursie at oracle.com]

Changeset: 1813adec7133
Author:    ihse
Date:      2012-04-26 16:19 +0200
URL:       http://hg.openjdk.java.net/build-infra/jdk8/rev/1813adec7133

If ALT_ variables are set, just warn about it, do not exit. But warn both at
beginning and end, so you don't miss it. :-)

! common/makefiles/Makefile