`rock_find_pkgconfig` does not restrict the search to whichever path is set within the pkg-config file

[doudou]

The find_library call in rock_find_pkgconfig is hinted with the paths found within the Cflags. This means it actually uses the paths that are provided by environment variables (or within the CMake code itself).

This works (albeit in a rather fragile way) when the paths are non-standard. It does not when the library that is being looked for lies in the system path but another version of it exists in the library search path.

Need to restrict the search path exactly to the paths that pkg-config lists in Cflags, or to the system paths if pkg-config returns no search paths.