Don't define wxUSE_FSWATCHER as 1 in configure if we can't implement it.
Also add a check to wx/unix/chkconf.h to verify that we didn't end up with an
inconsistent configuration somehow.
Closes#11670.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63306 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775