Don't assume prefix is either /usr or /usr/local

[intractabilis]

https://github.com/cginternals/glbinding/blob/9d32c2e21228db2c1892198ddb8e5a17cec7bddd/CMakeLists.txt#L161

A prefix can be anything, not necessarily either /usr or /usr/local. For example, it can be a Python virtual environment, with the proper bin, lib, and lib/cmake inside. The installation must proceed with the installation to these directories, not to the prefix root to the utter surprise of the user.