Provide CMake find module to locate opm-verteq
Use new style of prerequisite declaration

The prerequisites are now declared in their own file, so it can be
included by the main system. There is thus only one canonical place
where the list of prerequisites exist.
Support compiling files that are in the attic

Since these files are not actively maintained, we don't assume that
the project actually has any, nor are they part of the default build.
Move old files into the attic

The attic are the place where files which we don't use right now, but
which we acknowledge some amount of valuable time has been spent on and
which may be usable some day in the future, so we don't have the
conscience to right out delete them.

These files are not expected to compile, and certainly not to run and
produce sensible results, anymore. However, with some work they can
possibly be converted into proper unit tests or examples.
Issue warning about the status of files in the attic
Don't install debug syms for a library that isn't there
Only include library if there is one
Only make target if there is any source files

If there is no source files, we set a blank target to signal to the
other components of the build system that they shouldn't be looking
for a library.
Merge pull request #301 from rolk/301_verteq

Add find module for opm-verteq to build system
Merge pull request #302 from rolk/302_attic

Put old "non-unit-tests" in the attic instead
Merge pull request #303 from rolk/303_libless

Add support to the build system for building header-only modulesi
Merge remote-tracking branch 'upstream/master'