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
@@ -15,7 +15,7 @@ PACKAGE_ROOT_DIR = File.expand_path(File.join(File.dir
 REQUIRED_LIBXML_VERSION = "2.6.21"
 RECOMMENDED_LIBXML_VERSION = "2.9.3"
 
-REQUIRED_MINI_PORTILE_VERSION = "~> 2.8.0" # keep this version in sync with the one in the gemspec
+REQUIRED_MINI_PORTILE_VERSION = "~> 2.8.1" # keep this version in sync with the one in the gemspec
 REQUIRED_PKG_CONFIG_VERSION = "~> 1.1"
 
 # Keep track of what versions of what libraries we build against
@@ -1036,7 +1036,7 @@ 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", "nokogiri_gumbo.h")