configure: Check for stdio_ext.h

configure: Check for stdio_ext.h

For portability, check for stdio_ext.h during configure and define
HAVE_STDIO_EXT_H accordingly.

If the current system does not provide this header, use a fallback for
__fpending(). This definition will not work on all systems as it relies
on internal data structures of libc. A more portable solution should be
preferred, for example by using gnulib.