LLVM fixes

The first commit is because I ran into a problem backporting the LLVM 3.5 support to 2.2.10: Melbourne 2.1.0.0 is not C++11-compatible, and adding -std=c++11 to the system_cxxflags also causes it to apply to gems.

The second and third commit are for supporting the different configurations of shared libs: One lib when LLVM was built using autotools, many libs (matching the static ones) when built with cmake.