Avoid replacing 'struct timeval' and compiling lib/gettimeofday.c on MinGW.

Avoid replacing 'struct timeval' and  compiling lib/gettimeofday.c on MinGW.

 nt/mingw-cfg.site (gl_cv_sys_struct_timeval_tv_sec): Set to "yes"
 to avoid gnulib replacement of 'struct timeval' and the resulting
 compilation of lib/gettimeofday.c with incompatible version of
 gettimeofday.  Related discussions on emacs-devel:
 http://lists.gnu.org/archive/html/emacs-devel/2013-09/msg00286.html
 http://lists.gnu.org/archive/html/emacs-devel/2013-09/msg00361.html