gtk, docs: the module goes where GTK says, and the README says less

The install target guessed the module directory from whichever immodule
was already installed when pkg-config had nothing to say — a guess that
finds nothing on a fresh system and needed seven lines of README to
explain.  It uses GTK's own pkg-config variables now, or the
GTK_MODULE_DIR a packager sets.  The README also loses a negation about
a font argument that no longer exists and two paragraphs that repeat the
dependency list.

gtk/Makefile

@@ -7,13 +7,8 @@ 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_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)))
+GTK_MODULE_DIR ?= $(strip $(if $(GTK_BINARY_VERSION),\
+	$(GTK_LIBDIR)/gtk-3.0/$(GTK_BINARY_VERSION)/immodules))
 
 all: $(PROG)