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, quotes in configuration paths
Only regenerate config var file if actually changed
Remove make execution wrapper
Merge pull request #11 from rolk/011_cmakeupd

Update CMake scripts to commit e75d69ef from opm-core