Adapted FindOpenCL.cmake from cmake 3.1.0

1. FindOpenCL.cmake has been replaced by the module with the same name
from cmake 3.1.0. The module has some small adjustments.
Tested with cmake 2.8.8
2. Simplified gmxManageOpenCL.cmake, removed unused options.
Fixed build error for VS 2010
Fixed warnings when building against OpenCL 1.2