libgimp: libgimp does not depend on GTK+.

Only libgimpui depends on GTK+, display servers and other GUI-related
dependencies. There was a problematic include added in commit 0b56aa0d13 for
macOS, but the needed code (testing the macro GDK_WINDOWING_QUARTZ to use some
[NSApp activateIgnoringOtherApps:YES] API) doesn't seem to be present anymore in
there, so I think that removing this include (replace by including GLib for
other calls) should work fine. Of course, we'll know it when the separate CI
will test a macOS build as we still don't have in-Gitlab macOS jobs. :-/

libgimp/gimp.c

@@ -49,7 +49,7 @@
 #define WAIT_ANY -1
 #endif
 
-#include <gtk/gtk.h> /* need GDK_WINDOWING_FOO defines */
+#include <glib.h>
 
 #ifndef G_OS_WIN32
 #include "libgimpbase/gimpsignal.h"
@@ -68,10 +68,6 @@
 #include <sys/select.h>
 #endif
 
-#ifdef GDK_WINDOWING_QUARTZ
-#include <Cocoa/Cocoa.h>
-#endif
-
 #if defined(G_OS_WIN32) || defined(G_WITH_CYGWIN)
 #  ifdef STRICT
 #  undef STRICT

libgimp/meson.build

@@ -324,10 +324,11 @@ libgimp = library('gimp-'+ gimp_api_version,
   libgimp_sources,
   include_directories: rootInclude,
   dependencies: [
+    cairo,
+    gdk_pixbuf,
     gegl,
     gexiv2,
     drmingw,
-    gtk3,
   ],
   sources: [
     pdbgen
@@ -441,7 +442,6 @@ else
     'Gio-2.0',
     'GLib-2.0',
     'GObject-2.0',
-    'Gtk-3.0',
   ]
 
   gir_args = [
@@ -489,7 +489,7 @@ else
     symbol_prefix: [ 'gimp_ui', 'gimp' ],
     export_packages: 'gimp-ui-' + gimp_api_version,
     header: 'libgimp/gimpui.h',
-    includes: [ libgimp_gir_includes, libgimp_gir[0] ],
+    includes: [ libgimp_gir_includes, 'Gtk-3.0', libgimp_gir[0] ],
     extra_args: gir_args,
     include_directories: rootInclude,
     install: true,