* lib/stdio.in.h (popen): Declare it if the system lacks this function.
* lib/popen.c (popen): Provide alternate definition for native Windows.
* m4/popen.m4 (gl_FUNC_POPEN): Test if popen exists. Set HAVE_POPEN.
* m4/stdio_h.m4 (gl_STDIO_H_DEFAULTS): Initialize HAVE_POPEN.
* modules/popen (Depends-on, configure.ac): Update condition.
* modules/stdio (Makefile.am): Substitute HAVE_POPEN.
* doc/posix-functions/popen.texi: Mention that the MSVC problem is
fixed.