title: Error building OpenCV for Android arm64-v8a
body: <p>I am trying to build OpenCV 3.2 for android. When I tried to build it for the armeabi-v7a configuration I had no problems compiling it. However, when I tried to compile it for the arm64-v8a hardware I am getting the following message:  </p>
  
  <pre><code>/Library/Android/sdk/ndk-bundle/platforms/android-21/arch-arm/usr/include/signal.h:87:12: error: expected ')'
      void (*sa_sigaction)(int, struct siginfo*, void*);
             ^
  /Library/Android/sdk/ndk-bundle/platforms/android-21/arch-arm/usr/include/asm/signal.h:102:24: note: expanded from macro 'sa_sigaction'
  #define sa_sigaction _u._sa_sigaction
                         ^
  /Library/Android/sdk/ndk-bundle/platforms/android-21/arch-arm/usr/include/signal.h:87:10: note: to match this '('
      void (*sa_sigaction)(int, struct siginfo*, void*);
           ^
  /Library/Android/sdk/ndk-bundle/platforms/android-21/arch-arm/usr/include/signal.h:87:12: error: duplicate member '_u'
      void (*sa_sigaction)(int, struct siginfo*, void*);
             ^
  /Library/Android/sdk/ndk-bundle/platforms/android-21/arch-arm/usr/include/asm/signal.h:102:22: note: expanded from macro 'sa_sigaction'
  #define sa_sigaction _u._sa_sigaction
                       ^
  /Library/Android/sdk/ndk-bundle/platforms/android-21/arch-arm/usr/include/signal.h:86:18: note: previous declaration is here
      sighandler_t sa_handler;
                   ^
  /Library/Android/sdk/ndk-bundle/platforms/android-21/arch-arm/usr/include/asm/signal.h:101:20: note: expanded from macro 'sa_handler'
  #define sa_handler _u._sa_handler  
  </code></pre>
  
  <p>This is how cmake is configured:</p>
  
  <pre><code>cmake -DCMAKE_TOOLCHAIN_FILE=$ANDROID_TOOLCHAIN -DANDROID_NDK=$NDK_ROOT  -DANDROID_ABI=arm64-v8a  -DANDROID_NATIVE_API_LEVEL=21  -DCMAKE_INSTALL_PREFIX=~/opencv/arm64 -DBUILD_TESTS=OFF -DBUILD_PERF_TESTS=OFF -DBUILD_TIFF=false -DTIFF_LIBRARY=$TIFF_LIBRARY -DTIFF_INCLUDE_DIR=$TIFF_INCLUDE_DIR -DBUILD_OPENEXR=false  
  </code></pre>
  
  <p>Has anyone else encountered this issue? I have been struggling with it for two days now. Any help would be greatly appreciated.  </p>
  
  <p><strong>Edit</strong>: I noticed that cmake ouputs this line:  </p>
  
  <pre><code>Target:                      Android 21 armv7-a
  </code></pre>
  
  <p>even though I set the abi to arm64-v8a. Any ideas why?</p>