Mark the graphics interop toggles as options

This allows enabling/disabling them via CMake without editing the
CMakeLists.txt file.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 207b878..35b64b2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -38,9 +38,9 @@
 #-----------------------------------------------------------
 # Default Configurable Test Set
 #-----------------------------------------------------------
-set(D3D10_IS_SUPPORTED)
-set(D3D11_IS_SUPPORTED)
-set(GL_IS_SUPPORTED)
+option(D3D10_IS_SUPPORTED "Run DirectX 10 interop tests" OFF)
+option(D3D11_IS_SUPPORTED "Run DirectX 11 interop tests" OFF)
+option(GL_IS_SUPPORTED "Run OpenGL interop tests" OFF)
 
 #-----------------------------------------------------------
 # Vendor Customization