GTK libs on Windows and OS X

Windows

The official libs are available from http://win32builder.gnome.org/
We do not need all of them, the needed subset will be included in the dependency bundle (in buildwin/gtk)
The headers are in our tree (https://github.com/nohal/OpenCPN/commit/9cf849d218cdf983b9eacd029090406dbc255469)
Local build is a total alchemy. Do not even try...

The instructions come from https://wiki.gnome.org/Projects/GTK+/OSX/Building
Get https://git.gnome.org/browse/gtk-osx/plain/gtk-osx-build-setup.sh
sh gtk-osx-build-setup.sh
.local/bin/jhbuild bootstrap
- There should be no need to set the SDK versions /actually when I tried to set everything to 10.7, the builds started to get broken/ as 10.7 is the newest jhbuild deployment target available
- cmake failed, made it succeed by looking up the way it called ./bootstrap and then running it manually with added --system-curl parameter in another terminal, then repeated jhbuild build stage and everything completed fine
.local/bin/jhbuild build meta-gtk-osx-bootstrap
.local/bin/jhbuild build meta-gtk-osx-core
.local/bin/jhbuild build librsvg
The libs are now available in gtk/inst/lib
Get the list of ones we need using otool -L gtk/inst/lib/librsvg-2.2/dylib
Copy them to buildosx/gtk
We use the local includes (same as on Windows)
We can most likely provide the dylibs prebuilt to save people time and use consistent set of libraries