Commit f7bd00c1 pulled in a gnulib module that fails to compile
on mingw.  Work around it while waiting for an upstream gnulib fix.

* gnulib/local/lib/pty.in.h (openpty): Provide forward
declarations of opaque structs not present on mingw.
* gnulib/local/lib/openpty.c (openpty): Provide stub for mingw.