Moved OpenMP detection to separate file.

This cleans up the main CMakeLists.txt file a bit and
removes manual line breaking from the OpenMP warning
message. It also emphasizes the different parts
(compiler detection, finding package, and final setup)
that are part of the OpenMP detection in Gromacs.

Change-Id: Id13e555d8dac1b5e8bb9c07111a4546a3b438a55