modules: added comment explaining include path issues with SDL1/OSX.

modules/FindSDL2.cmake

@@ -43,6 +43,13 @@ endif()
 
 # Include dir
 find_path(SDL2_INCLUDE_DIR
+    # We must search file which is present only in SDL2 and not in SDL1.
+    # Apparently when both SDL.h and SDL_scancode.h are specified, CMake is
+    # happy enough that it found SDL.h and doesn't bother about the other.
+    #
+    # On OSX, where the includes are not in SDL2/SDL.h form (which would solve
+    # this issue), but rather SDL2.framework/Headers/SDL.h, CMake might find
+    # SDL.framework/Headers/SDL.h if SDL1 is installed, which is wrong.
     NAMES SDL_scancode.h
     PATH_SUFFIXES ${PATH_SUFFIXES}
 )