gcam.exe should use runtime search paths for Java jars

[bje-]

Currently, GCAM hard codes the JARS_LIB environment variable, set at compile time, into the gcam.exe executable via a preprocessor macro. This makes GCAM brittle if the jar files are moved around or if you only discover a configuration problem after GCAM has been compiled (incorrectly).

Wouldn't it be better to use the CLASSPATH environment variable at runtime to find any Java jars?