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.