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
Handle ERT not found condition gracefully

If the files for ERT is not found, then the find module cheats and
indicates that it didn't compile either (since there are some later
tests for HAVE_ERT).

However, erraneously, it gave this variable the value 0, which tests
for false in the CMake code but not in the #ifdef checks in the C++
code. This error is cancelled in opm-core by the fact that it didn't
store it as a cache variable, but resurfaced in opm-polymer when it
probes the second time (as both opm-core as a dependency and
opm-polymer itself declare a dependency on ERT).

It is now stored in a way that should be satisfactory to both
projects.
Merge pull request #10 from rolk/010_havecached

Error in config.h