gtk: find the module directory through the GTK runtime again

A module built in the container is installed on a host that has no GTK
development files, and there pkg-config knows nothing: dropping the
runtime query left `doas make install` with nowhere to put the module.
gtk-query-immodules-3.0 answers where GTK looks, so ask it when
pkg-config cannot, as before, and say so in the README.

gtk/Makefile

@@ -7,8 +7,14 @@ GTK_QUERY_IMMODULES ?= gtk-query-immodules-3.0
 GTK_LIBDIR ?= $(shell $(PKG_CONFIG) --variable=libdir gtk+-3.0 2>/dev/null)
 GTK_BINARY_VERSION ?= $(shell $(PKG_CONFIG) \
 	--variable=gtk_binary_version gtk+-3.0 2>/dev/null)
-GTK_MODULE_DIR ?= $(strip $(if $(GTK_BINARY_VERSION),\
-	$(GTK_LIBDIR)/gtk-3.0/$(GTK_BINARY_VERSION)/immodules))
+# Without GTK development files, ask the runtime where its modules live.
+GTK_RUNTIME_MODULE_DIR = $(shell GTK_PATH= GTK_IM_MODULE_FILE= \
+	$(GTK_QUERY_IMMODULES) 2>/dev/null | \
+	awk -F '"' '/^"\// { sub("/[^/]*$$", "", $$2); print $$2; exit }')
+GTK_MODULE_DIR ?= $(strip $(if \
+	$(and $(strip $(GTK_LIBDIR)),$(strip $(GTK_BINARY_VERSION))),\
+	$(GTK_LIBDIR)/gtk-3.0/$(GTK_BINARY_VERSION)/immodules,\
+	$(GTK_RUNTIME_MODULE_DIR)))
 
 all: $(PROG)