Fix android libc++ build

[guycook]

resolves #6511

This pullrequest changes

Presently, when using cmake -DANDROID_STL=c++_shared the build completes successfully but you can't actually use the generated artefacts in an android project that uses clang + libc++ due to linker errors. The changes in this PR resolve that by using the correct std library headers.

[vpisarev]

@mshabunin, could you please review it?

[alalek]

@guycook What Android NDK do you use?

[guycook]

@alalek I've tested and got it working with two separate ndk versions:

• r12_b-1 sourced from https://aur.archlinux.org/packages/android-ndk/
• r10e-rc4 as bundled with webrtc rev 979c268830 (this is why I need clang/libc++ btw)

To use the built library I'm using Android Studio + gradle-experimental:0.8.0-beta2 and a build script that looks like:

model {
    android {
        ndk {
            toolchain "clang"
            stl "c++_shared"
            abiFilters.add("armeabi-v7a")
            cppFlags.add("-std=c++11")
            cppFlags.add("-DNDEBUG")
            cppFlags.add("-DANDROID")
            cppFlags.add("-I" + localProperty('opencv.native') + "/jni/include")
            ldFlags.add("-L" + localProperty('opencv.native') + "/libs/armeabi-v7a")
            ldFlags.add("-L" + localProperty('opencv.native') + "/3rdparty/libs/armeabi-v7a")
            ldLibs.add("opencv_core")
            ldLibs.add("tegra_hal")
            ldLibs.add("tbb")
            ldLibs.add("zlib")
        }
    }
}

The relevant cmake flags are:

ANDROID_TOOLCHAIN_NAME=arm-linux-androideabi-clang3.5
ANDROID_STL=c++_shared
ANDROID_ABI="armeabi-v7a"
ANDROID_NATIVE_API_LEVEL=21
CMAKE_C_FLAGS_RELEASE="-Os -DNDEBUG -DANDROID -fvisibility=hidden -ffunction-sections -fstack-protector-all"
CMAKE_CXX_FLAGS_RELEASE="-Os -DNDEBUG -DANDROID -fvisibility=hidden -ffunction-sections -fstack-protector-all -fvisibility-inlines-hidden"
ENABLE_PRECOMPILED_HEADERS=OFF
CMAKE_TOOLCHAIN_FILE=platforms/android/android.toolchain.cmake

[mshabunin]

This statement should be moved to "comon.hpp", because some other source files in Carotene need it too (e.g. canny.cpp).

[mshabunin]

Thank you! Please squash both commits into one and we will merge it.

[guycook]

@mshabunin Cool, I think that's done now (I haven't ever squashed before)

[mshabunin]

👍