CMakeLists.txt: set dylib version numbers properly. (bug #2915.)

CMakeLists.txt

@@ -46,6 +46,9 @@ set(SDL_MICRO_VERSION 9)
 set(SDL_INTERFACE_AGE 0)
 set(SDL_BINARY_AGE 9)
 set(SDL_VERSION "${SDL_MAJOR_VERSION}.${SDL_MINOR_VERSION}.${SDL_MICRO_VERSION}")
+# the following should match the versions in Xcode project file:
+set(DYLIB_CURRENT_VERSION 10.0.0)
+set(DYLIB_COMPATIBILITY_VERSION 1.0.0)
 
 # Set defaults preventing destination file conflicts
 set(SDL_CMAKE_DEBUG_POSTFIX "d"
@@ -451,6 +454,8 @@ if(USE_GCC OR USE_CLANG)
 
   if(APPLE)
     list(APPEND EXTRA_LDFLAGS "-Wl,-undefined,error")
+    list(APPEND EXTRA_LDFLAGS "-Wl,-compatibility_version,${DYLIB_COMPATIBILITY_VERSION}")
+    list(APPEND EXTRA_LDFLAGS "-Wl,-current_version,${DYLIB_CURRENT_VERSION}")
   else()
     set(CMAKE_REQUIRED_FLAGS "-Wl,--no-undefined")
     check_c_compiler_flag("" HAVE_NO_UNDEFINED)