Separate build concerns for better maintainability

Introduce a main build file which uses modules in the cmake/ directory
for most of its bulk work, which agains retrieve information from files
in the root of the source tree (dune.module, CMakeLists_files.cmake) to
get information about the project.

Thus, the cmake/ directory is shared between all the projects down to
the last bit; only project-specific customizations go into the main file
in form of _hook macros (which are called in specific places).
Put project-specific information in dune.module

This information was previously stored in the build system; now they
are all in one canonical place!
Further classify source files into languages

Other modules (notably the precompiled header one) may set properties
directly on the source file; we need to separate them into categories
for which language they belong to.
Safe-guard against version mismatch

The version declared for the build system (in dune.module) is checked
against what is in the source (e.g. opm/core/version.h) and if these
don't match, then issue an author warning.

This will help us keep the two version numbers in sync., since it
becomes very obvious when we don't. The benefit for this is to not have
the build system start mucking with the code just to dump some static
information in there.
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.
Issue warning about the status of files in the attic
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.
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.
Provide a define for having OpenMP
Enable support for Ninja build generator

Use the Ninja build generator if '--enable-ninja' is passed on the
command line to ./configure (or of course if -GNinja is passed
directly to CMake).
Recognize Clang to be a GCC-compatible compiler

Clang aims to be compatible with GCC when it comes to command-line
parameters. Where we enable functionality based on the presence of
a GCC-compiler, we can use the same functionality with Clang.

This patch introduces a new variable CXX_COMPAT_GCC which is true if
the compiler handles the same options as GCC, and this variable is
subsequently used in tests instead of CMAKE_COMPILER_IS_GNUCXX (which
remains if we need to test if we really have GCC, e.g. for version)
Have distclean target keep up with build improvements

If you run `cmake --build . -- distclean` all generated files will
now be gone! This adds the files that have been introduced in later
improvements to the build system.
Build system: make it work for modules which do not need libraries
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.
Allow PCH option to be on even if there is no library