GTK2/3: Fix warnings calling pthread_setspecific() with a non-pointer value using gcc 11. Also update the readme with similar information on dw_exec() as I had previous added for Mac. Renamed _locked_by_me to _dw_locked_by_me used by the thread safety macros to show these are internal DW variables.

[Desktop Entry]
Name=Dynamic Windows Test
Exec=@prefix@/bin/dwtest 
Terminal=false
MultipleArgs=false
Type=Application
Categories=Application;Development;X-Red-Hat-Base;