Make pkg-config functional again

When writing opm-core.pc, the list of libraries is basically dumped to file. This works fine if everything is in system locations, but if you have a local build it fails because the command-line syntax of the linker doesn't take absolute paths.

After this patch, one can (again) compile small test programs with:

    export PKG_CONFIG_PATH=/path-to/opm-core-build:$PKG_CONFIG_PATH
    g++ foo.cpp $(pkg-config --cflags --libs opm-core)

This change should only affect those using pkg-config based builds (which probably at the time isn't any amongst the core developers since the error has gone undetected)