win: Make build able to find python3 on Windows and Linux

Use the py launcher on Windows which understands she bangs
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 762919a..25a91f6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -35,6 +35,11 @@
     if(NOT EXISTS ${LUNARGLASS_PREFIX})
         message(FATAL_ERROR "Necessary LunarGLASS components do not exist: " ${GLSLANG_PREFIX})
     endif()
+    if (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+        set(PYTHON_CMD "python3")
+    endif()
+else()
+        set(PYTHON_CMD "py")
 endif()
 
 # loader: Generic XGL ICD loader