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

[Erik Joelsson]

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

/Erik