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

[Paul Eggert]

My guess is that the FreeBSD 7 host has a stray /usr/include/spawn.h. 
There shouldn't be such a file, but if you try to upgrade to 8.x and 
then downgrade back to 7.x the file is left behind, and this messes up 
configuration.  Try removing the stray file.  Or, if you don't have root 
access, try './configure ac_cv_header_spawn_h=no'.  For more details, 
please see <https://forums.freebsd.org/viewtopic.php?&t=15775>.

Since FreeBSD 7 isn't being maintained any more, this would be low 
priority for us to fix even if it were a bug in m4 or gnulib.