Introduced gb_INSTROOT, which is the same as $(INSTDIR) except for Mac OS X,
where it is $(INSTDIR)/LibreOffice.app/Contents.  Most stuff ends up there (so
most occurrences of $(INSTDIR) have been replaced with $(gb_INSTROOT)), but SDK-
related stuff goes to $(INSTDIR)/$(gb_Package_SDKDIRNAME).  (And
GeneratedPackage needed to be made more flexible, to allow for packages that go
into either of those two places.)

For Android and iOS, gb_INSTROOT probably still needs to be set.

The most obvious missing thing yet to make instdir work for Mac OS X is the
instdir/*/LibreOffice.app/Contents/ure/ vs.
instdir/*/LibreOffice.app/Contents/ure-link/ split.

Change-Id: I4478edd27b14c92c96d92d5169bdca3ec50d78f5