fpending: port to recent Cygwin change to stdio_ext.h

Reported by LRN in
<http://lists.gnu.org/archive/html/bug-gnulib/2013-08/msg00028.html>.
* lib/fpending.h: Don't worry about HAVE_DECL___FPENDING;
just declare __fpending unless it's a macro.
A duplicate decl shouldn't hurt.
* m4/fpending.m4 (gl_FUNC_FPENDING): Check that an __fpending
call compiles and links, instead of separately checking for
decl and lib function.
* modules/fpending (configure-ac):
Adjust to fpending.m4's renaming of shell variable.