Allow to specify ruby binary; Small tweak of build flags

[falkTX]

No description provided.

[falkTX]

using -L/usr/local/lib is not safe for cross-compilation.
removed -g because I don't think it's useful there (you can use it in your own private CXXFLAGS)

allowing to specify ruby binary simplifies quite a lot of things from the MOD side,
and won't hurt normal usage.

[x37v]

Looks good, thanks!