Let ruby load the devkit instead of trying to find it ourselves.

Rakefile

@@ -21,7 +21,7 @@ task :checkout do
 end
 
 desc "compile v8 via the ruby extension mechanism"
-task :compile do
+task :compile => :devkit do
   sh "ruby ext/libv8/extconf.rb"
 end
 
@@ -77,5 +77,15 @@ task :clean do
   sh "cd #{GYP_Source} && git checkout -f && git clean -dxf"
 end
 
+task :devkit do
+  begin
+    if RUBY_PLATFORM =~ /mingw/
+      require "devkit"
+    end
+  rescue LoadError => e
+    abort "Failed to activate RubyInstaller's DevKit required for compilation."
+  end
+end
+
 task :default => [:checkout, :compile, :spec]
 task :build => [:clean, :checkout]

appveyor.yml

@@ -18,7 +18,6 @@ environment:
 # scripts that run after cloning repository
 install:
   - set PATH=C:\Ruby%ruby_version%\bin;%PATH%
-  - C:\Ruby%ruby_version%\DevKit\devkitvars.bat
   - git submodule update --init
   - ruby --version
   - gem --version