Synchronise build system with OPM-Core

This change-set brings in the most recent changes to the OPM build system from opm-core.  Notably, this propagates the opm-parser requirements into `opm-porsol` which fixes the recent spate of errors on the automatic builders.

Tested on (Debug and Release modes):
 * Ubuntu 12.04 LTS x86_64 (CMake 2.8.7, GCC 4.6.3, Boost 1.46.1)
 * CentOS 5.10 x86_64 (CMake 2.8.10.1, GCC 4.4.7, Boost 1.48)