MinGW patch for OpenCV 3.3.0

mingw.patch

From 37163b5ad31d1a7dffa3d4fdf65ce9a357d12054 Mon Sep 17 00:00:00 2001
From: Amro <amroamroamro@gmail.com>
Date: Fri, 18 Aug 2017 16:19:26 +0300
Subject: [PATCH] fixes for MinGW

- add a define before including DShow header,
  to not use secure string replacements not defined by mingw
- dont pass gcc flags to windres which doesnt understand them
  (another workaround is to set ENABLE_PRECOMPILED_HEADERS=OFF)

References:
- DShow fix: https://trac.ffmpeg.org/ticket/999
- windres fix: https://github.com/mxe/mxe/pull/1485
- https://wiki.qt.io/How_to_setup_Qt_and_openCV_on_Windows#OpenCV
---
 cmake/OpenCVPCHSupport.cmake      | 9 +++++++++
 modules/videoio/src/cap_dshow.cpp | 1 +
 2 files changed, 10 insertions(+)

diff --git a/cmake/OpenCVPCHSupport.cmake b/cmake/OpenCVPCHSupport.cmake
index 12e804d90..373f01ad9 100644
--- a/cmake/OpenCVPCHSupport.cmake
+++ b/cmake/OpenCVPCHSupport.cmake
@@ -31,6 +31,15 @@ ELSE()
     SET(PCHSupport_FOUND FALSE)
 ENDIF()
 
+IF(CMAKE_COMPILER_IS_GNUCXX AND MINGW)
+    # in case of MinGW, we drop <FLAGS> because windres doesnt understand the
+    # various GCC flags like -Winvalid-pch or -include
+    # (it only accepts the -D and -I flags, i.e <DEFINES> and <INCLUDES>)
+    # https://github.com/Kitware/CMake/blob/master/Modules/Platform/Windows-windres.cmake
+    # https://sourceware.org/binutils/docs-2.28/binutils/windres.html
+    set(CMAKE_RC_COMPILE_OBJECT "<CMAKE_RC_COMPILER> -O coff <DEFINES> <INCLUDES> <SOURCE> <OBJECT>")
+ENDIF()
+
 MACRO(_PCH_GET_COMPILE_FLAGS _out_compile_flags)
 
     STRING(TOUPPER "CMAKE_CXX_FLAGS_${CMAKE_BUILD_TYPE}" _flags_var_name)
diff --git a/modules/videoio/src/cap_dshow.cpp b/modules/videoio/src/cap_dshow.cpp
index d8a9e8441..2b9c1c0b2 100644
--- a/modules/videoio/src/cap_dshow.cpp
+++ b/modules/videoio/src/cap_dshow.cpp
@@ -108,6 +108,7 @@ Thanks to:
 #include <vector>
 
 //Include Directshow stuff here so we don't worry about needing all the h files.
+#define NO_DSHOW_STRSAFE
 #include "DShow.h"
 #include "strmif.h"
 #include "Aviriff.h"
-- 
2.14.1