Cleanup icons Makefile

Replace full-subdirs with a single Makefile for all icons, making
things both simpler and more concise.

Also, properly use Automake variable prefixes instead of doing it
manually.

configure.ac

@@ -742,32 +742,6 @@ data/atril.appdata.xml.in
 data/atril.desktop.in
 data/Makefile
 data/icons/Makefile
-data/icons/16x16/Makefile
-data/icons/16x16/apps/Makefile
-data/icons/16x16/actions/Makefile
-data/icons/16x16/mimetypes/Makefile
-data/icons/22x22/Makefile
-data/icons/22x22/apps/Makefile
-data/icons/22x22/actions/Makefile
-data/icons/22x22/mimetypes/Makefile
-data/icons/24x24/Makefile
-data/icons/24x24/apps/Makefile
-data/icons/24x24/actions/Makefile
-data/icons/24x24/mimetypes/Makefile
-data/icons/32x32/Makefile
-data/icons/32x32/actions/Makefile
-data/icons/32x32/mimetypes/Makefile
-data/icons/48x48/Makefile
-data/icons/48x48/apps/Makefile
-data/icons/48x48/actions/Makefile
-data/icons/64x64/Makefile
-data/icons/64x64/apps/Makefile
-data/icons/128x128/Makefile
-data/icons/128x128/apps/Makefile
-data/icons/scalable/Makefile
-data/icons/scalable/apps/Makefile
-data/icons/scalable/actions/Makefile
-data/icons/scalable/mimetypes/Makefile
 help/Makefile
 help/reference/Makefile
 help/reference/libdocument/Makefile