RFR: JDK-8235728: JDK-8212780 breaks builds with a custom X11 include path

[Alexey Semenyuk]

Please review fix [2] for jpackage bug [1].

- adds $(X_CFLAGS) to compiler command line.

Patch contributed by Arthur Eubanks (aeuba...@google.com).

- Alexey

[1] https://bugs.openjdk.java.net/browse/JDK-8235728

[2] http://cr.openjdk.java.net/~asemenyuk/8235728/webrev.00