Make subsurface compile with -DGSEAL_ENABLE

This forces us to use the proper gtk accessor functions.  It may not be
worth it if people actually do the Qt conversion, but if we want to try
gtk3 at some point, this might help.

This all came about because I was trying to explain on G+ what an
immense pain this all was to even figure out, if you don't actually know
gtk at all.  Google and the gtk migration guide are almost useless, and
the gtk2 documentation itself actually uses the fields directly without
any accessor functions in several places.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Let's call them GTKCFLAGS, not GTK2CFLAGS

Maybe they will be for GTK3 some day.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>