libfortran/90038 Reap dead children when wait=.false.

When using posix_spawn or fork to launch a child process, the parent
needs to wait for the child, otherwise the dead child is left as a
zombie process. For this purpose one can install a signal handler for
SIGCHLD.

2019-05-19  Janne Blomqvist  <jb@gcc.gnu.org>

	PR libfortran/90038
	* intrinsics/execute_command_line (sigchld_handler): New function.
        (execute_command_line): Install handler for SIGCHLD.
        * configure.ac: Check for presence of sigaction and waitpid.
        * config.h.in: Regenerated.
        * configure: Regenerated.

Regtested on x86_64-pc-linux-gnu.

From-SVN: r271384