install the onelab.py module to the right directory
The current(gmsh 2.8.3) CMake File installs the onelab python module to /usr/bin where it doesn't belong, as it is neither executable nor is /usr/bin contained in the Python search path for modules. Using Pythons distutils tools would make it easier for people who compile it themselves as well as for packagers to put everything in the right place and get a clean setup. http://docs.python.org/2/distutils/index.html