This is deprecated in newer automake versions, and this causes warnings
with automake 1.14:
panels/printers/Makefile.am:3: warning: 'INCLUDES' is the old name for
AM_CPPFLAGS' (or '*_CPPFLAGS')
https://bugzilla.gnome.org/show_bug.cgi?id=732189
This makes loading faster, with less I/O, avoids unnecessary
code duplication (around 1k lines shaved), and ensures that
all the panels link and work appropriately.
By the same token, it will stop external panels from being
created, and loaded.
https://bugzilla.gnome.org/show_bug.cgi?id=690036