Fix linker error (on Mac OS 10.5.8)

[lmjohns3]

I've been getting the following linker error when trying to npm install node-taglib :

i686-apple-darwin9-g++-4.0.1: -install_name only allowed with -dynamiclib

I finally tracked down the code in node-gyp that was generating the linker arguments in the makefiles, and it looks like it just needed augmenting with a -dynamiclib in the LDFLAGS.

[nikhilm]

On my snow leopard system with g++ 4.2.1 it works without the flag. But I'm merging this since adding the flag doesn't seem to cause any problems on my system.