Fix libsvm name for Darwin

[carlocab]

On macOS, shared libraries are expected to be installed with the naming convention

lib$NAME.$SOVERSION.dylib

We also prefix the library install name with @rpath to indicate to the dynamic linker that it should use RPATH to look for libsvm. Without this, the dynamic linker will only search the calling process' current working directory, which will, in general, not contain libsvm.