fix opencl explore for meson configure on non opencl machines

hysop/tools/opencl_explore.py

@@ -29,6 +29,15 @@ try:
     import pyopencl as cl
 except ModuleNotFoundError:
     cl = None
+except ImportError:
+    print("""PyOpenCL import error turned into this Warning. 
+This can be produced in kind of 'cross-installing' if configure on machine where PyOpenCL is not usable (like frontal on clusters).
+Otherwise, check your installation.
+
+In case of 'cross-installing' OpenCL default id for platform and device my be set by default to -1.
+User should change index at runtime exporting HYSOP_DEFAULT_DEVICE_ID and HYSOP_DEFAULT_PLATFORM_ID
+""",file=sys.stderr)
+    cl = None
 
 
 def size_human(s):