The common compiler and linker flags control, among other things, the
default visibility of symbols; without them, we leak symbols that ought
to be private.
We can build the name of the input and output files for the Wayland
protocols we use from the protocol name, stability, and version. This is
similar to how the autotools build does it, except much more clear and
without shelling out twice to sed just to resolve the Makefile rule.
gdkprivate-wayland.h includes generated wayland client protocol
headers and is included from gdkdisplaymanager.c, so we need to
generate those client protocol headers first also when building
main gdk itself.
This is how it's done in the autotools build. Also avoids problems
with multiple source files having the same name (gdkeventsource.c).
Also move broadway backend code into broadway subdir.
