OpulenZ/opl2 build fixes (Apple)

- Cleaned up many non-bracketed sections for stricter compilers.
- Remove custom `INLINE` keyword when building on clang.
- Renable opl2 via plugins\CMakeLists.txts

https://cloud.githubusercontent.com/assets/6345473/2919394/c474d668-d6da-11e3-82db-778a7ce6014d.png