RFR: JDK-8163102: Fix headless only configuration option

[Erik Joelsson]

Hello,

The current --disable-headful option is weird. Even worse, it doesn't 
work since the makefiles still uses the legacy BUILD_HEADLESS_ONLY 
variable, which configure never touches.

In this patch I'm replacing the configure option with 
--enable-headless-only. Default is disabled, where both headless and GUI 
support is built. When enabled, only headless support is built. These 
are the two variants we are ever interested in.

On the makefile side, I redefined the variable as 
ENABLE_HEADLESS_ONLY=true/false.

I also needed to change jawt.c to get a headless only build to actually 
work. The requirement that -DJAVASE_EMBEDDED is set seems unnecessary 
and unrelated to me.

Bug: https://bugs.openjdk.java.net/browse/JDK-8163102

Webrev: http://cr.openjdk.java.net/~erikj/8163102/webrev.01

/Erik