npth_sigmask and npth_sigwait are not symbols, so
-Wl,--no-undefined-version will report errors. Actually npth_sigwait has
no definition, so just remove it. The npth_sigmask macro seems not
useful either, so remove it as well.
* src/npth.h.in: Remove npth_sigmask and npth_sigwait.
* w32/npth.h: Likewise.
* src/libnpth.vers: Likewise.
symbols to fix -Wl,--no-undefined-version