Fix Darwin build.

CMake doesn't have a `MACOS` variable (see the full list here: https://cmake.org/cmake/help/latest/manual/cmake-variables.7.html#variables-that-describe-the-system).

The easiest workaround is to simply assume that if the operating system is an Apple one but not iOS, it must be MacOS. If finer distinction is necessary, `CMAKE_SYSTEM_NAME` value can be used (https://cmake.org/cmake/help/latest/variable/CMAKE_SYSTEM_NAME.html#variable:CMAKE_SYSTEM_NAME).

The breakage was introduced by cr/289100860.

PiperOrigin-RevId: 289516285

Author: Googler

release_build_files/CMakeLists.txt

@@ -31,7 +31,7 @@ if(ANDROID)
 elseif(APPLE)
   if(IOS)
     set(FIREBASE_SDK_LIBDIR ${FIREBASE_CPP_SDK_DIR}/libs/ios/universal)
-  elseif(MACOS)
+  else() #Assume MacOS
     set(FIREBASE_SDK_LIBDIR ${FIREBASE_CPP_SDK_DIR}/libs/darwin/universal)
   endif()
 elseif(MSVC)