system.h (STDIN_FILENO): Provide default definition if one is not provided by the system header files.

        * system.h (STDIN_FILENO): Provide default definition if one is not
        provided by the system header files.
        (STDOUT_FILENO, STDERR_FILENO): Likewise.
        * i386/xm-djgpp.h (COLLECT2_HOST_INITIALIZATION): New macro.
        * collect2.c (main): Use it.
        (pexecute_pid): New variable. Holds return value from call to pexecute.
        (collect2_execute): Rework to use pexecute instead of fork.
        (collect2_wait): Use pwait() instead of wait().

Co-Authored-By: Jeffrey A Law <law@cygnus.com>

From-SVN: r25960