Due to the same problem with sizeof(long) mismatch as in the previous
commit, we can't use struct timeval, with its long fields, in 64 bit
Cygwin builds, and need to use __ms_timeval instead.
Add wxTimeVal_t type to hide this difference and update all code
compiled under MSW (there is no need to uglify Unix-only code using
timeval, as in wxSelectDispatcher, for example) to use it instead of
timeval.
ebb06fbeaf