Add build-level support for TF2

This compiles fine as-is.

AMBuildScript

@@ -20,6 +20,7 @@ WinLinuxMac = ['windows', 'linux', 'mac']
 
 PossibleSDKs = {
   'css':  SDK('HL2SDKCSS', '2.css', '6', 'CSS', WinLinuxMac, 'css'),
+  'tf2':  SDK('HL2SDKTF2', '2.tf2', '12', 'TF2', WinLinuxMac, 'tf2'),
 }
 
 def ResolveEnvPath(env, folder):

AMBuilder

@@ -10,7 +10,7 @@ project = SM.HL2Project(builder, projectName + '.ext')
 project.sources += [
   'extension.cpp',
   'ringbuffer.cpp',
-  '../../public/smsdk_ext.cpp'
+  os.path.join(Extension.sm_root, 'public', 'smsdk_ext.cpp'),
 ]
 
 for sdk_name in SM.sdks: