Create a set of config variables for implementation

The config variables in _CONFIG_IMPL_VARS are only written to config.h
and not the .pc or -config.cmake files. They can thus only be used by
the implementation files and not the headers. Use this set to avoid
exposing the variables to other projects, when it is likely that a
probe used by that other project may turn up a different result.
Provide hook to configure getting other's version

This hook together with the bundled convenience macro makes it
possible to add the version of another module to config.h using
code like this:

macro (config_hook)
  opm_need_version_of ("dune-common")
endmacro (config_hook)
Give template for adding version requirement
Probe DUNE module version number from installation

Figure out where the closest dune.module is, and parse it for
information. The defines are added to config.h, so that our code
may behave differently for various versions available.

It actually works for opm-core 1.0 as well, but don't tell anyone;
we rather want people to use opm/core/version.h instead.

If a module has been backported and installed to a directory which
also contains the version number, e.g. /usr/include/dune-2.3, to
avoid clashing with an existing version in /usr/include, then this
part is now stripped from the include directory and added back to
the lib directory where we are looking for dune.module
Make available version numbers for these modules
Fix bug in determining if directory is found

The parenthesis is to make it less ambiguous how the boolean expression
should be evaluated.
Module mode CMake probe for opm-material
Upgrade Findopm-material to use separate prerequisite list

This will now be included both by the find module and by the project
file itself.
Merge pull request #311 from rolk/311_material

Find module for opm-material
Allow PCH option to be on even if there is no library
Only build before library target if there is any
Only configure libtool for libraries that exist
Merge pull request #312 from rolk/312_nolib

Allow several options to be used even if we don't have a library
Merge pull request #298 from rolk/298_dunever

Make version numbers for DUNE prerequisites available