[cairo] Making configure error message more clear

[Bill Spitzak]

>   - If we only use $PREFIX/lib/pkgconfig if PKG_CONFIG_PATH is not set, then
> user setting PKG_CONFIG_PATH to another location will break the build since
> they now have to add $PREFIX/lib/pkgconfig manually.

I don't understand what is wrong with this solution. If they are setting 
it, then they have probably set it correctly, thus it either includes 
this directory, or that directory is not needed.