Build fix & simplification

I had issues building the project because I'm building with clang as default C++ compiler. In the process of fixing that, I also found a couple of other issues that I believe are not necessary for a working and neat build.

In terms of CMAKE_CXX_FLAGS, they should not be specific to GNUCXX as that excludes other compilers which support C++11 as well. The "-g -O" options are better passed from externally, i.e. with the appropriate environment variables passed to the cmake executable.

Other stuff seems like it's leftovers from previous iterations of this project that doesn't make a whole lot of sense to keep anymore.