The current directory structure is as follows:
install/make/au2.0 which builds windows AU. With Mac AU coming along, we need to restructure this directory structure as follows:

install/make/au/windows -> Windows AU files go here
install/make/au/macosx -> Mac OS X AU files/scripts go here
ADD install/make/au/Makefile -> Builds the appropriate AU depending on the $(PLATFORM) value of the build
MODIFY install/make/Makefile -> Build AU only for mac and windows
MODIFY install/common/Defs-windows.gmk -> Add the AU output directory variable for windows
MODIFY install/common/Defs-macosx.gmk -> Add the AU output directory variable(s) for macosx

Additional tasks linked to this CR:

Right now, MacOS makefile structure is designed such that it can build the installer. We want to add another directory hierarchy (i.e. create a parent for the macos installer makefile) so that the Makefiles are:

a) More modular; making it easier for enhancement/future project builds
b) The builds for each individual future projects for macos are independent (separation of concerns) and hence easier to maintain. Also, not to mention lesser merge conflicts.