execute.c doesn't have fallback for systems without posix_spawn

[Garrett Cooper]

Hi,
    I recently tried to install m4 1.4.17 on FreeBSD 7 (which doesn't
have posix_spawn) and I ran into build errors with execute.c; it was
looking for posix_spawn-related constants and the API, which didn't
exist.
    I'm not sure what the best course of action would be for this --
either remove the autoconf-related feature checking or add the
necessary #defines to support OSes without posix_spawn.
Thanks!
-Garrett