cmake: Move cxx flag testing/setup into its own file.

- Move CXX flags stuff into build/cxx_flags.cmake.
- include CMake modules where they're used.
- Add ENABLE_WERROR flag-- enables warnings as errors.
- Add -Wshorten-64-to-32 and -Wnarrowing to CXX flags.

Change-Id: I5d93a39a3eff6ae81f9e094927b8f0cb9f36bbb0

CMakeLists.txt

@@ -7,10 +7,8 @@
 ##  be found in the AUTHORS file in the root of the source tree.
 cmake_minimum_required(VERSION 3.2)
 project(LIBWEBM CXX)
-include("${CMAKE_CURRENT_SOURCE_DIR}/build/msvc_runtime.cmake")
 
-include(CheckCXXCompilerFlag)
-include(CheckCXXSourceCompiles)
+include("${CMAKE_CURRENT_SOURCE_DIR}/build/msvc_runtime.cmake")
 
 set(LIBWEBM_SRC_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
 
@@ -18,27 +16,18 @@ set(LIBWEBM_SRC_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
 option(ENABLE_WEBMTS "Enables WebM PES/TS support." ON)
 option(ENABLE_TESTS "Enables tests." OFF)
 option(ENABLE_IWYU "Enables include-what-you-use support." OFF)
+option(ENABLE_WERROR "Enable warnings as errors." OFF)
 set(GTEST_SRC_DIR "${LIBWEBM_SRC_DIR}/../googletest" CACHE PATH
     "Path to Googletest git repository.")
 
 # This directory is where libwebm will build googletest dependencies.
 set(GTEST_BUILD_DIR "${CMAKE_BINARY_DIR}/googletest_build")
 
+include("${CMAKE_CURRENT_SOURCE_DIR}/build/cxx_flags.cmake")
 if (ENABLE_TESTS OR ENABLE_WEBMTS)
   include("${CMAKE_CURRENT_SOURCE_DIR}/build/cxx11_tests.cmake")
 endif ()
 
-# Set warning levels.
-if (MSVC)
-  set(CMAKE_CXX_FLAGS "/W4 ${CMAKE_CXX_FLAGS}" CACHE STRING "" FORCE)
-  # Disable MSVC warnings that suggest making code non-portable.
-  set(CMAKE_CXX_FLAGS "/wd4996 ${CMAKE_CXX_FLAGS}" CACHE STRING "" FORCE)
-else ()
-  set(CMAKE_CXX_FLAGS "-Wall -Wextra ${CMAKE_CXX_FLAGS}" CACHE STRING "" FORCE)
-  set(CMAKE_CXX_FLAGS "-Wno-deprecated ${CMAKE_CXX_FLAGS}" CACHE
-      STRING "" FORCE)
-endif ()
-
 set(CMAKE_CXX_FLAGS "-D__STDC_CONSTANT_MACROS ${CMAKE_CXX_FLAGS}")
 set(CMAKE_CXX_FLAGS "-D__STDC_FORMAT_MACROS ${CMAKE_CXX_FLAGS}")
 set(CMAKE_CXX_FLAGS "-D__STDC_LIMIT_MACROS ${CMAKE_CXX_FLAGS}")
@@ -201,4 +190,3 @@ if (ENABLE_IWYU)
     message(STATUS "  See README.libwebm for more information.")
   endif ()
 endif ()
-

build/cxx11_tests.cmake

@@ -7,6 +7,9 @@
 ##  be found in the AUTHORS file in the root of the source tree.
 cmake_minimum_required(VERSION 3.2)
 
+include(CheckCXXCompilerFlag)
+include(CheckCXXSourceCompiles)
+
 if (CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang" OR
     CMAKE_CXX_COMPILER_ID STREQUAL "Clang" OR
     CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
@@ -134,5 +137,3 @@ if (NOT HAVE_UNIQUE_PTR
   set(ENABLE_WEBMTS OFF)
   message(WARNING "C++11 feature(s) not supported, tests and webmts disabled.")
 endif ()
-
-

build/cxx_flags.cmake

@@ -0,0 +1,37 @@
+##  Copyright (c) 2016 The WebM project authors. All Rights Reserved.
+##
+##  Use of this source code is governed by a BSD-style license
+##  that can be found in the LICENSE file in the root of the source
+##  tree. An additional intellectual property rights grant can be found
+##  in the file PATENTS.  All contributing project authors may
+##  be found in the AUTHORS file in the root of the source tree.
+cmake_minimum_required(VERSION 3.2)
+
+include(CheckCXXCompilerFlag)
+
+function (add_cxx_flag_if_supported cxx_flag)
+  unset(FLAG_SUPPORTED CACHE)
+  CHECK_CXX_COMPILER_FLAG("${cxx_flag}" FLAG_SUPPORTED)
+  if (FLAG_SUPPORTED)
+    set(CMAKE_CXX_FLAGS "${cxx_flag} ${CMAKE_CXX_FLAGS}" CACHE STRING "" FORCE)
+  endif ()
+endfunction ()
+
+# Set warning levels.
+if (MSVC)
+  add_cxx_flag_if_supported("/W4")
+  # Disable MSVC warnings that suggest making code non-portable.
+  add_cxx_flag_if_supported("/wd4996")
+  if (ENABLE_WERROR)
+    add_cxx_flag_if_supported("/WX")
+  endif ()
+else ()
+  add_cxx_flag_if_supported("-Wall")
+  add_cxx_flag_if_supported("-Wextra")
+  add_cxx_flag_if_supported("-Wno-deprecated")
+  add_cxx_flag_if_supported("-Wshorten-64-to-32")
+  add_cxx_flag_if_supported("-Wnarrowing")
+  if (ENABLE_WERROR)
+    add_cxx_flag_if_supported("-Werror")
+  endif ()
+endif ()