Fix the linker complaining about the lib being renamed

If the .a and .dylib files are renamed, an executable linked with the renamed file still tries to find the library using the original name, and crashes.

With this PR, two variants are created:

* `libjson_linux-gcc-4.2.1_libmt.a`
* `libjson_linux-gcc-4.2.1_libmt.dylib`
* `libjsoncpp.a` (symlink to `libjson_linux-gcc-4.2.1_libmt.a`)
* `libjsoncpp.dylib` (symlink to `libjson_linux-gcc-4.2.1_libmt.dylib`)