hg: panama/panama: Disable libclang if not available

[henry.jen at oracle.com]

Changeset: 785099bf7560
Author:    henryjen
Date:      2014-11-12 15:26 -0800
URL:       http://hg.openjdk.java.net/panama/panama/rev/785099bf7560

Disable libclang if not available

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