[CMake] Use find_package config mode for GLM

[ljrk0]

GLM requires using the new config mode as it only ships a
glmConfig.cmake.

more info:
http://stackoverflow.com/q/34634850/4453524
https://cmake.org/cmake/help/latest/command/find_package.html

fixes https://github.com/danhedron/openrw/issues/41

[ljrk0]

[REVERTED]
I've also added Config mode for Bullet, since it does support it too.

[ljrk0]

We could also remove cmake/modules/FindSFML.cmake as it is already provided by upstream:
https://github.com/SFML/SFML/blob/master/CMakeLists.txt#L297

[ljrk0]

Just noticed that Bullet, although providing a Config cmake seems not to build properly that way. I'll investigate.

[ljrk0]

Okay, deleted the commit. FindBullet.cmake is already provided by CMake itself, so we gonna use that. The Config way it doesn't set up include paths properly.

[Bulding completes now]

[Hockenberry]

Using patch (fe0c93f) under ubuntu 16.04 LTS does work with a glm rebuild from source.

[danhedron]

Thanks.