Factor out routine that converts lib list to cmdline

This routine is useful in its own rights and should be made available as
a function that others can call too.
Write cmdline in .pc file without abs paths

When passing libraries to gcc/ld, the search path and library name must
be specified separately. This is already done when writing a libtool .la
and should be done in the pkg-config .pc file too.
Remove exclusion erroneously copied from opm-core

The exclude statement correctly belongs in the Doxylocal file, fixed
in commit 3c7594d in opm-core. However, this file was copied from that
project before the fix.
Add more support for configuring MPI

Handles options --enable-mpi and --with-mpi-prefix for compatibility
with other autotools projects which uses these settings to tell the
build system where the local MPI library is.
Merge pull request #47 from rolk/047_cmake

CMake rollup