Add modular x11 dependencies.

Fixes Homebrew/homebrew#13638.

Library/Homebrew/dependencies.rb

@@ -64,7 +64,13 @@ private
     when :autoconf, :automake, :bsdmake, :libtool
       # Xcode no longer provides autotools or some other build tools
       Dependency.new(spec.to_s) unless MacOS::Xcode.provides_autotools?
-    when :x11, :libpng
+    when :libpng, :freetype, :pixman, :fontconfig, :cairo
+      if MacOS.lion_or_newer?
+        MacOS::XQuartz.installed? ? X11Dependency.new(tag) : Dependency.new(spec.to_s)
+      else
+        X11Dependency.new(tag)
+      end
+    when :x11
       X11Dependency.new(tag)
     else
       raise "Unsupported special dependency #{spec}"