Output deprecated warning for --rezflags and permanently disable it.

Output deprecated warning for --rezflags and permanently disable it.
Remove WXCONFIG_POSTLINK_COMMAND that existed only for it.
Make the explanation more explanatory.  This is not an invitation
for stray options and script helpers to do a little 'open mike',
its a holding ground for nasty things we still need to fix.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@36293 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775