Include required headers into pth.h.
* src/npth.h.in: Include sys/time.h and signal.h. Include time.h only if available. * configure.ac (INSERT_SYS_TIME_H, INSERT_TIME_H, INSERT_SIGNAL_H): Detect header files and ac_subst snippets.
sys/time.h is required for struct timeval and signal.h for sigset_t.
Without that them the regression test fails on IRIX. Conditional
inclusion of these files might not bee required, but it should not
harm, either.