SetupJdkLibrary should setup SRC and -I flags automatically

We should separate the -I flags (location of header files) from other flags, and generate these automatically, wherever possible. Since we have a standard way of locating native code, and most libraries just want to have an -I path to their own source base and the generated Java header, we should be able to provide this in SetupJdkLibrary.

Relatedly, SetupJdkLibrary should also be able to figure out the SRC for a library, given that it follows the usual location convention.