RFR: JDK-8163102: Fix headless only configuration option

[Tim Bell]

Erik:

> 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

Looks good to me.

/Tim