Expose INCLUDEPATH and LIBS qmake variables

This allows one to specify the include and lib paths for gl and zlib.

Example:

    gem install capybara-webkit -- \
      --with-gl-dir=/nix/store/1sw1cyny213ih9dpdsq8h2kwqaqcm6vp-mesa-10.2.9 \
      --with-zlib-dir=/nix/store/cb649pfdf14335d07jcfmsik7a1rsgbf-zlib-1.2.8

Fixes #695

extconf.rb

@@ -1,2 +1,20 @@
+require "mkmf"
+
+$CPPFLAGS = ""
+
+dir_config("gl")
+dir_config("zlib")
+
+include_path = $CPPFLAGS.gsub("-I", "").strip
+libs = $LIBPATH.map { |path| "-L#{path}"}.join(" ").strip
+
+unless include_path.empty?
+  ENV["CAPYBARA_WEBKIT_INCLUDE_PATH"] = include_path
+end
+
+unless libs.empty?
+  ENV["CAPYBARA_WEBKIT_LIBS"] = libs
+end
+
 require File.join(File.expand_path(File.dirname(__FILE__)), "lib", "capybara_webkit_builder")
 CapybaraWebkitBuilder.build_all

lib/capybara_webkit_builder.rb

@@ -1,5 +1,6 @@
 require "fileutils"
 require "rbconfig"
+require "shellwords"
 
 module CapybaraWebkitBuilder
   extend self
@@ -52,8 +53,10 @@ module CapybaraWebkitBuilder
     success
   end
 
-  def makefile(config = '')
-    sh("#{qmake_bin} -spec #{spec} #{config}")
+  def makefile(*configs)
+    configs += default_configs
+    configs = configs.map { |config| config.shellescape}.join(" ")
+    sh("#{qmake_bin} -spec #{spec} #{configs}")
   end
 
   def qmake
@@ -82,6 +85,19 @@ module CapybaraWebkitBuilder
     end
   end
 
+  def default_configs
+    configs = []
+    libpath = ENV["CAPYBARA_WEBKIT_LIBS"]
+    cppflags = ENV["CAPYBARA_WEBKIT_INCLUDE_PATH"]
+    if libpath
+      configs << "LIBS += #{libpath}"
+    end
+    if cppflags
+      configs << "INCLUDEPATH += #{cppflags}"
+    end
+    configs
+  end
+
   def build_all
     makefile &&
     qmake &&