build: Prefix SDL- to revision from REVISION.txt

This makes it possible to get the version number of an unknown SDL binary
from `strings libSDL3.so.0 | grep SDL-`, like we could for SDL 2.

Resolves: https://github.com/libsdl-org/SDL/issues/14114
Signed-off-by: Simon McVittie <[email protected]>

CMakeLists.txt

@@ -3638,7 +3638,8 @@ endforeach()
 if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/REVISION.txt")
   file(STRINGS "${CMAKE_CURRENT_SOURCE_DIR}/REVISION.txt" revisions)
   list(GET revisions 0 revisions_0)
-  string(STRIP "${revisions_0}" SDL_REVISION)
+  string(STRIP "${revisions_0}" revisions_0_stripped)
+  set(SDL_REVISION "SDL-${revisions_0_stripped}")
 else()
   set(SDL_REVISION "" CACHE STRING "Custom SDL revision (only used when REVISION.txt does not exist)")
 endif()