For compatibility with other m4 implementations, sysval returns
signal<<8 rather than 0 if syscmd is terminated by a signal.
* configure.ac (AC_CHECK_HEADERS_ONCE): Check for sys/wait.h.
* src/builtin.c (include): Include sys/wait.h when stdlib.h does
not provide wait macros.
(WTERMSIG, WIFSIGNALED, WIFEXITED): More fallback macros.
(M4SYSVAL_EXITBITS, M4SYSVAL_TERMSIGBITS): New helper macros.
(m4_esyscmd): Set sysval to -1 on failure.
(m4_sysval): Print 127 on failure, and accomodate signals if they
are detectable.
* NEWS: Document this change.
* doc/m4.texinfo (Platform macros, Esyscmd, Sysval): Fix typos in
last commit.
* checks/check-them: Likewise.
* src/debug.c (debug_set_file): Work around mingw fstat bug.