gnu: Use HTTPS on gtkmm.org home pages.

* gnu/packages/gtk.scm (atkmm, gtkmm)[home-page]: Use HTTPS.
* gnu/packages/gnome.scm (libgnomecanvasmm)[home-page]: Likewise.
* gnu/packages/glib.scm (glibmm)[home-page]: Likewise.

gnu/packages/glib.scm

@@ -535,7 +535,7 @@ has an ease of use unmatched by other C++ callback libraries.")
     (propagated-inputs
      `(("libsigc++" ,libsigc++)
        ("glib" ,glib)))
-    (home-page "http://gtkmm.org/")
+    (home-page "https://gtkmm.org/")
     (synopsis "C++ interface to the GLib library")
     (description
      "Glibmm provides a C++ programming interface to the part of GLib that are

gnu/packages/gnome.scm

@@ -1595,7 +1595,7 @@ creating interactive structured graphics.")
     (native-inputs
      `(("gtkmm-2" ,gtkmm-2)
        ("pkg-config" ,pkg-config)))
-    (home-page "http://gtkmm.org")
+    (home-page "https://gtkmm.org")
     (synopsis "C++ bindings to the GNOME Canvas library")
     (description "C++ bindings to the GNOME Canvas library.")
     (license license:lgpl2.0+)))

gnu/packages/gtk.scm

@@ -1043,7 +1043,7 @@ library.")
     (native-inputs `(("pkg-config" ,pkg-config)))
     (propagated-inputs
      `(("glibmm" ,glibmm) ("atk" ,atk)))
-    (home-page "http://www.gtkmm.org")
+    (home-page "https://www.gtkmm.org")
     (synopsis "C++ interface to the ATK accessibility library")
     (description
      "ATKmm provides a C++ programming interface to the ATK accessibility
@@ -1088,7 +1088,7 @@ toolkit.")
                         ;; Don't fail because of the missing /etc/machine-id.
                         (setenv "DBUS_FATAL_WARNINGS" "0")
                         #t))))))
-    (home-page "http://gtkmm.org/")
+    (home-page "https://gtkmm.org/")
     (synopsis
      "C++ interface to the GTK+ graphical user interface library")
     (description