Allow variables from indirect dependencies to bubble up

The previous implementation was a function, which although OK from an
implementation standpoint -- the local variables doesn't pollute the
global namespace -- would not allow variables that were set in indirect
dependencies to bubble up to the main module. This is a problem for
modules which are dependent on configuration variables to be present.
Abstract into common version and parameterize
Use our own find routine recursively

Don't use bare find_package in OpmPackage when we have already written
OpmFind to take out the worst warts.
Merge pull request #201 from rolk/201_transitive

Allow variables from indirect dependencies to bubble up
Add MPI dependency for dune-common

mpihelper.hh needs to know HAVE_MPI in the same way as the compiled
library in order to generate a consistent interface.
Only use MPI if explicitly enabled

Running with MPI on a regular workstation will probably not increase
performance, so this should be disabled as default.
Enable MPI support from DUNE with --enable-parallel
Bugfix change from macro to function

All variables we introduce which is not parameters are in fact global
variables that may be overwritten by recursive invocation!
Bugfix expand variable definition
Search in dunecontrol build directories for other modules
Bugfix don't search for module twice!
Merge pull request #202 from rolk/202_mpi

Allow MPI to be found and opm-core configured the same way as DUNE
Allow precompiled headers to be enabled from command-line
Use find modules for libraries with extra processing

Some libraries require more information than what is present in the
xxx-config.cmake file, e.g. the caller must know whether HAVE_TUPLE
is available and probably used when compiling dune-common, and put
this in its own config.h file.

Code to take care of these variables must therefore be in the client
configuration, and this is the same code which is used to handle the
autotools version, namely the find module, so a practical solution
is to just revert to that in both cases.
Don't search for source in build directories
Search for libraries in dunecontrol's default build dir
Merge pull request #204 from rolk/204_precomp

Allow precompiled headers to be enabled from command-line