sdl: fix sdl_config prefix detection.

Deals with the issue flagged up in this commit:
https://github.com/Homebrew/homebrew/commit/0d6d0213dc4cc406cdfdcb5ba742b361a0ab2210

Closes #34721.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
sdl: update 1.2.15 bottle.