Move all common build code to cmake/ directory

This changeset move the CMake code that is common most of the OPM projects out of CMakeLists.txt and into a separate file in the cmake/ subdirectory. It also extracts the declaration of prerequisites into separate files (one for each project), so that the duplication in the find modules can be removed. In addition, the configure script is also moved to the cmake/ directory, leaving just a small wrapper in-place.

The advantage of this is that it should be possible to keep the cmake/ directory 100% identical between the various modules, and still have all changes to the build system (may there be few of them!) localized there. The disadvantage is that there is now one more level of include to get to the buik of the code.

One question regarding the prerequisite list is whether it is worth to have them in a separate file or if the find modules should be dumped, relying solemnly on config mode.

If these changes are deemed acceptable, similar changes will be done to the other projects; I want to discuss the principles here first, though.