app: add some CPPFLAGS in core/Makefile.am to fix the build on OSX

cherry-picked and adapted to gimp-2-8 from
00cd9b4e47dea4b4f51da60cb03fa2d575dcdb88

Resolved conflicts:
	app/core/Makefile.am
configure: Also search for the -gtk2 version of mac integration

If a suffixed version is not found it will still fall back to
searching for the old version.

Cherry-picked and adapted to gimp-2-8 from
f6533db8b739b95ca5d71c2d5ac2b28c54c650bf

Resolved conflicts:
	configure.ac
OS X build: update build instructions in README

Update gtk-mac-bundler preparations. Add line to run gtk-mac-bundler in
a JHBuild shell. Add gtk-mac-bundler instructions for building
gimp-master.