configure: Fix FD_ISSET test under -Werror

The uninitialized variable warning from T x; broke the test if the
user passed CXXFLAGS='-O2 -Wall -Werror'.  (Users shouldn’t do that;
our own --enable-compile-warnings=error option was unaffected.)

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

configure.ac

@@ -384,7 +384,7 @@ class T {
 public:
   fd_set fds;
   bool Fun( void ) const { return FD_ISSET( 0, &fds ); } };]],
-[[T x; return x.Fun();]])],
+[[T x = T(); return x.Fun();]])],
   [AC_DEFINE([FD_ISSET_IS_CONST], [1],
      [Define if FD_ISSET() fd_set argument is const.])
    AC_MSG_RESULT([yes])],