Replace FindFTGL.cmake with FTGLConfig.cmake

[mathstuf]

Since FTGL is creating this file, it can just tell CMake where it lives rather than shipping a file on how to find itself.

[ulrichard]

When I created the file, I think finder scripts were still the way to do it. I later read that people should switch to the Config scripts, but never did one so far.
As I don't actively use FTGL at the moment, I don't know when I'll find time. But pull requests are always welcome.