Allow different C/C++ standards when this is used as a subproject.

Inspired by https://github.com/google/snappy/pull/85

PiperOrigin-RevId: 279649967

CMakeLists.txt

@@ -6,15 +6,21 @@ cmake_minimum_required(VERSION 3.9)
 # Keep the version below in sync with the one in db.h
 project(leveldb VERSION 1.22.0 LANGUAGES C CXX)
 
-# This project can use C11, but will gracefully decay down to C89.
-set(CMAKE_C_STANDARD 11)
-set(CMAKE_C_STANDARD_REQUIRED OFF)
-set(CMAKE_C_EXTENSIONS OFF)
-
-# This project requires C++11.
-set(CMAKE_CXX_STANDARD 11)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_CXX_EXTENSIONS OFF)
+# C standard can be overridden when this is used as a sub-project.
+if(NOT CMAKE_C_STANDARD)
+  # This project can use C11, but will gracefully decay down to C89.
+  set(CMAKE_C_STANDARD 11)
+  set(CMAKE_C_STANDARD_REQUIRED OFF)
+  set(CMAKE_C_EXTENSIONS OFF)
+endif(NOT CMAKE_C_STANDARD)
+
+# C++ standard can be overridden when this is used as a sub-project.
+if(NOT CMAKE_CXX_STANDARD)
+  # This project requires C++11.
+  set(CMAKE_CXX_STANDARD 11)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON)
+  set(CMAKE_CXX_EXTENSIONS OFF)
+endif(NOT CMAKE_CXX_STANDARD)
 
 if (WIN32)
   set(LEVELDB_PLATFORM_NAME LEVELDB_PLATFORM_WINDOWS)