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
Allow uppercase alternatives for package location vars
Fix bug with whitespace at end of env.var.
Don't add anything if no precompiled header is specified
Only regenerate config var file if actually changed
Remove make execution wrapper
Conditionally disable example if compiler is not C++11
Find OpenMP in a reusable way
Use OpenMP in this project

The source code have #pragma omp, so we should enable this from the
command line. (But why isn't it inherited from opm-core?)
Merge pull request #20 from rolk/020_cmakeupd

Update CMake scripts to commit 11bf7bf3 from opm-core