Allow to search for multiple libraries in package

This functionality is (probably) necessary for opm-parser:

Some packages are split up in several library files, where each of them doesn't have their own CMake find-module. So we cannot put this list in the prerequisites. Unlike the headers, where we just add the directory,
all the libraries must be explicitly named on the link line. Thus, we should allow to specify more than one name in the lib parameter.

The first name specified is designated as the "primary" library and is assigned to the _LIBRARY variable; the others occur in _LIBRARIES as if they were prerequisites.

Note that the build system cannot replicate this setup itself; it always assumes that we are building (at most) one library from the source.
files.