List files that are needed to build the library

This list will be picked up by the modules at a later commit.
Specify source files explicitly rather than with glob
Various improvements in the build process

Allow uppercase alternatives, fix whitespace bug, allow empty pre-
compiled header, find OpenMP
Only regenerate config var file if actually changed
Remove make execution wrapper
Merge pull request #28 from rolk/028_cmakeupd

Update CMake scripts to commit 11bf7bf3 from opm-core