Fix compilation of unittests under Clang.

It seems the GTest header file in LLVM 3.3 (and possibly other versions)
makes use of typeid() but the build system passes -fno-rtti. These
are incompatible and if building with Clang then compilation will
fail. GCC doesn't seem to care!
Merge pull request #81 from delcypher/fix_unittest_llvm33_clang

Fix compilation of unittests under Clang.