try to include CL/cl_ext.h to get CL_PLATFORM_NOT_FOUND_KHR

configure.ac

@@ -570,12 +570,14 @@ AC_DEFUN([STARPU_CHECK_OPENCL],
         fi
     fi
 
-    CPPFLAGS="${SAVED_CPPFLAGS}"
-    LDFLAGS="${SAVED_LDFLAGS}"
-
     if test "$have_valid_opencl" = "yes" -a "$__opencl_include_dir" != "no"; then
         STARPU_OPENCL_CPPFLAGS="-I$__opencl_include_dir"
+	AC_CHECK_HEADERS([CL/cl_ext.h])
     fi
+
+    CPPFLAGS="${SAVED_CPPFLAGS}"
+    LDFLAGS="${SAVED_LDFLAGS}"
+
     if test "$have_valid_opencl" = "yes" ; then
         if test "$__opencl_lib_dir" != "no"; then
             STARPU_OPENCL_LDFLAGS="-L$__opencl_lib_dir"

src/drivers/opencl/driver_opencl_utils.c

@@ -29,6 +29,10 @@
 #include "driver_opencl_utils.h"
 #include "driver_opencl.h"
 
+#ifdef HAVE_CL_CL_EXT_H
+#include <CL/cl_ext.h>
+#endif
+
 char *_starpu_opencl_program_dir;
 
 #define _STARPU_STRINGIFY_(x) #x