Introduce extension Macros

Implement idea from #248
to add an version macro to the extension headers so that
users can guard application code using the macro to ensure the correct
APIs are being used.

Extensions can then increment the version when they change the APIs
without breaking user code.

See
[CL_MAKE_VERSION](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#CL_MAKE_VERSION)
for how the macro version is defined.

CL/cl.h

@@ -113,8 +113,8 @@ typedef cl_bitfield         cl_device_atomic_capabilities;
 typedef cl_bitfield         cl_device_device_enqueue_capabilities;
 typedef cl_uint             cl_khronos_vendor_id;
 typedef cl_properties       cl_mem_properties;
-typedef cl_uint             cl_version;
 #endif
+typedef cl_uint             cl_version;
 
 typedef struct _cl_image_format {
     cl_channel_order        image_channel_order;
@@ -914,8 +914,6 @@ typedef struct _cl_name_version {
 /* cl_khronos_vendor_id */
 #define CL_KHRONOS_VENDOR_ID_CODEPLAY               0x10004
 
-#ifdef CL_VERSION_3_0
-
 /* cl_version */
 #define CL_VERSION_MAJOR_BITS (10)
 #define CL_VERSION_MINOR_BITS (10)
@@ -939,8 +937,6 @@ typedef struct _cl_name_version {
    (((minor) & CL_VERSION_MINOR_MASK) << CL_VERSION_PATCH_BITS) | \
    ((patch) & CL_VERSION_PATCH_MASK))
 
-#endif
-
 /********************************************************************************************************/
 
 /* CL_NO_PROTOTYPES implies CL_NO_CORE_PROTOTYPES: */