Support cross compile for windows via mingw64.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1170

Change-Id: I8430600ceb610fbf1ecff34f0a2d2c1a57627692

README.libwebm

@@ -118,4 +118,26 @@ standard include files occur, one solution can be found here:
 https://github.com/include-what-you-use/include-what-you-use/issues/100
 
 
+CMake cross compile
+To cross compile libwebm for Windows using mingw64 run cmake with the following
+arguments:
+
+$ cmake -DCMAKE_TOOLCHAIN_FILE=path/to/libwebm/build/mingw64_toolchain.cmake \
+  path/to/libwebm
+
+Note1: As of this writing googletest will not build via mingw64 without
+disabling pthreads.
+googletest hash: d225acc90bc3a8c420a9bcd1f033033c1ccd7fe0
+
+To build with tests when using mingw64 use the following arguments when running
+CMake:
+
+$ cmake -DCMAKE_TOOLCHAIN_FILE=path/to/libwebm/build/mingw64_toolchain.cmake \
+  -DENABLE_TESTS=ON -Dgtest_disable_pthreads=ON path/to/libwebm
+
+Note2: i686-w64-mingw32 is the default compiler. This can be controlled using
+the MINGW_PREFIX variable:
+
+$ cmake -DCMAKE_TOOLCHAIN_FILE=path/to/libwebm/build/mingw64_toolchain.cmake \
+  -DMINGW_PREFIX=x86_64-w64-mingw32 path/to/libwebm
 

build/mingw64_toolchain.cmake

@@ -0,0 +1,23 @@
+##  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.
+
+set(MINGW_PREFIX "i686-w64-mingw32" CACHE STRING "MinGW cross compiler")
+
+# Target OS name.
+set(CMAKE_SYSTEM_NAME Windows)
+
+# Cross compiler values.
+set(CMAKE_C_COMPILER ${MINGW_PREFIX}-gcc)
+set(CMAKE_CXX_COMPILER ${MINGW_PREFIX}-g++)
+set(CMAKE_RC_COMPILER ${MINGW_PREFIX}-windres)
+
+# Tell CMake not to look too hard for things; we want the target toolchain
+# defaults.
+set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

common/file_util.cc

@@ -20,7 +20,7 @@
 namespace libwebm {
 
 std::string GetTempFileName() {
-#ifndef _MSC_VER
+#if !defined _MSC_VER && !defined __MINGW32__
   char temp_file_name_template[] = "libwebm_temp.XXXXXX";
   int fd = mkstemp(temp_file_name_template);
   if (fd != -1) {