Avoid warnings about enabling fp64 in CL1.2, copied from PyOpenCL

loopy/library/preamble.py

@@ -43,7 +43,9 @@ def default_preamble_generator(seen_dtypes, seen_functions):
 
     if has_double:
         yield ("00_enable_double", """
+            #if __OPENCL_VERSION__ <= CL_VERSION_1_1
             #pragma OPENCL EXTENSION cl_khr_fp64: enable
+            #endif
             """)
 
     if has_complex: