tests run by "make install" fail if generators are disabled

configuring with generators disabled (configure --disable-gen-java, for example) produces a failed build because the tests run by "make install" require the java, cpp, rb, perl, csharp, and js generators.

I would personally favor either removing those --disble-gen options altogether, or adding a warning message in the configure --help output that disabling those generators is not recommended/will cause test failure.