Some simple changes. Mostly implementing Mr. Sada Kenji patches to
JX x11-toolkit family ports:
1. nonexistent -> ${NONEXISTENT}
2. Allow you to define WRKDIRPREFIX safely.
-JXPORT_WRKSRC= ${JXPORT}/work/JX-1.5.3
+JXPORT_WRKSRC!= cd ${JXPORT} && ${MAKE} -V WRKSRC