cmake: Several fixes to the build system

[ghost]

• Install the FindGLM cmake script so it will work out of the box for downstream users
• Don't build the tests unless testing is enabled; this removes the need to abort cmake (which prevents running "make install" or any other targets)
• Use standard install paths

I'm not familiar with the cmake git branches, so if this needs rebasing onto any other branches, just let me know.

[Groovounet]

The proposed changes looks great and will makes it for GLM 0.9.5.4 branch,

Thanks for contributing!
Christophe

[JPGygax68]

I'm working under Windows, using GLM version 0.9.5.4. I think FindGLM.cmake gets installed to the wrong directory here; it goes to ${CMAKE_INSTALL_PREFIX}/lib/cmake, where CMake can't find it by itself. According to http:https://www.cmake.org/cmake/help/v3.0/command/find_package.html, the suffix lib/cmake is valid for Linux, but not for Windows.