Further macOS adjustments

Created by: fxcoudert

The use of gtkosxapplication.h is limited to Gtk3 (where it was deprecated, see https://wiki.gnome.org/Projects/GTK/OSX/Integration). It should be guarded with the macro MAC_INTEGRATION, as per the documentation (https://jralls.github.io/gtk-mac-integration/GtkosxApplication.html); that macro is defined by pkg-config when necessary.

The remaining OSX variable in Makefile is not necessary. macOS is now a special case of LINUX=1 (like FreeBSD and other Unix-like platforms), which is much easier for maintenance.