Add -I flag position to work with already installed gumbo.h.

Index: ext/nokogiri/extconf.rb
--- ext/nokogiri/extconf.rb.orig
+++ ext/nokogiri/extconf.rb
@@ -954,7 +954,7 @@ libgumbo_recipe = process_recipe("libgumbo", "1.0.0-no
     end
   end
 end
-append_cppflags("-I#{File.join(libgumbo_recipe.path, "include")}")
+$CPPFLAGS = "-I#{File.join(libgumbo_recipe.path, "include")} #{$CPPFLAGS}".dup
 $libs = $libs + " " + File.join(libgumbo_recipe.path, "lib", "libgumbo.a")
 $LIBPATH = $LIBPATH | [File.join(libgumbo_recipe.path, "lib")]
 ensure_func("gumbo_parse_with_options", "gumbo.h")