Fix flags so that they work for mac and non-mac.

I think that you can use, for instance, a "CONFIG -= -O2" to remove a flag ("-O2") that *might* already be present in the "default" set that the mkspecs files bring in.  On Windows it actually defaults to having the "debug-and-release" entry in the CONFIG variable (running qmake then parses the project file **three** times which can be a bit confusing for messages that get generated thrice!) but only builds the last specified (release | debug) type unless "build_all" is also present - see the QMake manual in the QtCreator help, look for the help on qmake "Variables".  **I** put in the -O0 to explicitly turn off any optimisation because I understand it makes it easier to debug the code if gdb doesn't have to step thorough code that has been rearranged by optimisations (though it does tend to make the executable larger).