Added support for building on linux.

Also added building of 'test/main.cpp' to CMakeLists.txt;
I'm not really sure how this ever built for anyone before, if I were to
propose a suggestion it would be through the original authors (perhaps
unintentional) complete abuse of how Windows deals with path case
sensitivity, in both MSVC and CMake.

I've also added cmake scripts to find libsfml and libmodplug, for
systems where it is installed, and not only expecting to find the
libraries in ${CMAKE_BINARY_DIR}.