fix multilib devel conflict (rhbz#2031783)
    
    During build, npth rewrites its config script and header file with the
    current arch. This leads to a multilib file-conflict for npth-devel.
    
    Patch the config script to use `uname -m` and the header file comment to
    indicate both archs on x86.
    
    Effectively, this is a lean implementation of:
    https://fedoraproject.org/wiki/PackagingDrafts/MultilibTricks