fence: move pkg-config fail to bin target
    
    Currently make (GNU Make 4.2.1) will fail if pkg-config fails even if
    it's not necessary, e.g. make clean target. We move the make bailout if
    pkg-config failed to the binary target when pkg-config result is
    necessary.
    
    Note: this is using $(.SHELLSTATUS) which is only available on GNU Make
    version 4.2 or above.