when cross-compiling. The key to success is then you properly setup
  PKG_CONFIG_PATH before invoking configure.

  I also improved how NSS is detected by trying nss-config if pkg-config isn't
  present, and as a last resort just use the lib name and force the user to
  setup the LIBS/LDFLAGS/CFLAGS etc properly. The previous last resort would
  add a range of various libs that would almost never be quite correct.