CMake: Cleanup / Remove special handling / make more reuseable

If the user wants to compile with dietlibc, he can either do it manually by setting the CMAKE_C_COMPILER variable to the appropriate wrapper (wherever it's installed), or if the captcha is used as part of a larger project structure, the parent CMakeLists.txt file can take care of this if needed.

Let's keep this very simple program as simple as can be. Arguably, even CMake (or any build system at all, even make) is overkill, but using it makes it easer to integrate with other CMake projects.

In addition, the -std=gnu99 option isn't actually needed, I believe this was just copied from a different project. So this has been removed, too.

Using ${PROJECT_SOURCE_DIR} for the source files is probably a good idea in case someone wants to use CMake's add_subdirectory (which I don't recommend but is common practice).