FindDevIL: Restore IL_FOUND result variable

The fix in commit v3.8.0-rc1~257^2~1 (FindDevIL: fail properly when
library is not found, 2016-11-24) removed the previously-provided
`IL_FOUND` result variable.  Set it for compatibility and update the
documentation to mention the new variable.

Fixes: #16881

Modules/FindDevIL.cmake

@@ -25,7 +25,7 @@
 #                     library interfaces with OpenGL. It is not strictly needed
 #                     in applications.
 #    IL_INCLUDE_DIR - where to find the il.h, ilu.h and ilut.h files.
-#    IL_FOUND -       this is set to TRUE if all the above variables were set.
+#    DevIL_FOUND    - this is set to TRUE if all the above variables were set.
 #                     This will be set to false if ILU or ILUT are not found,
 #                     even if they are not needed. In most systems, if one
 #                     library is found all the others are as well. That's the
@@ -70,3 +70,5 @@ find_library(ILU_LIBRARIES
 FIND_PACKAGE_HANDLE_STANDARD_ARGS(DevIL DEFAULT_MSG
                                   IL_LIBRARIES ILU_LIBRARIES
                                   IL_INCLUDE_DIR)
+# provide legacy variable for compatiblity
+set(IL_FOUND ${DevIL_FOUND})