Make sure CMake is re-run if dune.module changes

Version number in this module is included in the name of the library;
if we make changes to dune.module, it should do a complete reconfigure.
Strings must contain ONLY numbers to be integers

The old pattern matched if there was an integer (digit) in the string;
it would have to be all non-digits to be quoted.
Add a label field to the project information

The difference between the version number (1.0) and the label (2013.03)
is: The version is time-independent but shows the level of compatibility,
see e.g. <http://www.semver.org>, whereas the label is a marketing tool
which describes the project progress.
Read the label from project info
Write version information into header file

Akin to config.h, we write project-version.h whenever the VCS sha hash
of the project changes. This file can then be included to embed this
into the project. Since this changes more frequently, we choose another
file than config.h. Care is also taken to not rewrite the header if the
information doesn't change, but the project is rebuilt.
Include version information in the library

This version information can be queried programmatically by using the
expored opm_core_version symbol.
Make sure that version info is updated before building code

Otherwise, the library may be declared "done" before the version number
process have had any change to touch files it depends upon. This ensures
that the version number is correct before we start building the library.
Don't update version info in Debug mode

It gets tiresome to rebuild the project everytime one makes changes to
any of the unit tests. We don't likely publish results based on the
debug code anyway, so tracking the version number there is not such a
pressing issue.
Merge pull request #290 from rolk/290_version

Embed version string into the library
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' into devel
Probe again later if ERT isn't found now

If we write a blank value into the cache, we tell CMake that ERT isn't
found and it won't check again. If we delete it from the cache in case
we didn't find the directories, then CMake will run the find module
again the next time we configure. Just probing for directories is cheap
so this is no problem. This allows us to install ERT, do a reconfigure
and it will then be picked up by the build system.

If we find ERT and it doesn't compile properly, a blank value will still
be written, and it won't try again the next time, so you must wipe the
cache if you *upgrade* ERT into a newer and more compatible version.
Merge pull request #291 from rolk/291_ertdep

Probe again later if ERT isn't found now
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' into devel