CMake: Fix SOVERSION/SONAME of non-release versions

When building SDL2 from git with CMake, you got libSDL2-2.0.so.1
instead of .0 (as it's the case when building with autotools).
This was caused by using LT_REVISION instead of LT_MAJOR for SOVERSION.

fixes #4310

CMakeLists.txt

@@ -2530,7 +2530,7 @@ if(SDL_SHARED)
   elseif(UNIX AND NOT ANDROID)
     set_target_properties(SDL2 PROPERTIES
       VERSION ${LT_VERSION}
-      SOVERSION ${LT_REVISION}
+      SOVERSION ${LT_MAJOR}
       OUTPUT_NAME "SDL2-${LT_RELEASE}")
   else()
     if(WINDOWS OR CYGWIN)