Heads up --enable-headless-only

[Gary Adams]

In the latest sync from jdk9 b138 there was change in the autoconfig
command line options. In the earlier mobile/dev  build instructions you 
could use
--disable-headful to bypass building of the desktop artifacts from compact3
profile. The new option is called --enable-headless-only.  This option 
was not
completely compatible with what was built for ios and android, so an 
additional
check was added libraries.m4 to bypass the need for X11 headers for
these embedded builds.