libfortran/90038: Use posix_spawn instead of fork

libfortran/90038: Use posix_spawn instead of fork

fork() semantics can be problematic.  Most unix style OS'es have
posix_spawn which can be used to replace fork + exec in many cases.
For more information see
e.g. https://www.microsoft.com/en-us/research/uploads/prod/2019/04/fork-hotos19.pdf

This replaces the one use of fork in libgfortran with posix_spawn.

2019-05-17  Janne Blomqvist  <jb@gcc.gnu.org>

        PR libfortran/90038
        * configure.ac (AC_CHECK_FUNCS_ONCE): Check for posix_spawn.
        * intrinsics/execute_command_line (execute_command_line): Use
        posix_spawn.
        * Makefile.in: Regenerated.
        * config.h.in: Regenerated.
        * configure: Regenerated.

Regtested on x86_64-pc-linux-gnu.

From-SVN: r271340