RFR: JDK-8157348: Build fails with certain source configurations

[Tim Bell]

Erik:

> Several recent build changes and source moves have made certain subset 
> configurations of the source fail to build. Some minor tweaks are 
> needed to correct this.
>
> * Building of gtest needs to be conditional on the test sources being 
> available. Also adding --disable-gtest to be able to turn it off 
> explicitly.
> * When modifying module-infos, we already filter on the target module 
> of an export existing. Now also filter on the package being exported 
> existing.
> * When creating images, filter the set of JRE modules on the ones 
> actually being built. (for the JDK modules, the ALL_MODULES variable 
> has already been filtered using MODULES_FILTER so no need to redo it.)
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8157348
> Webrev: 
> http://cr.openjdk.java.net/~erikj/8157348/webrev.top.01/index.html

Looks good to me.

/Tim