wechat_qr: disable iconv dependancy for mingw

[berak]

resolves #2862

this also collapses all #if defined(XXX) blocks to disable iconv support into a single one

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• [x ] I agree to contribute to the project under Apache 2 License.
• [x ] To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• [x ] The PR is proposed to proper branch
• [x ] There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[berak]

uff, how can OpenCV CN Ubuntu 18.04 x86-64 fail on goturn tracking tests using OpenCV(3.4.14-pre) ? pr was against master branch ...

[alalek]

Thank you!

[ferdnyc]

Hmm. I was just about to open a PR to enable iconv in wechat_qrcode, since in my testing it builds fine if linked with mingw-w64-{x86_64,i686}-libiconv. And CMake has, ever since 3.11, included a FindIconv.cmake in the standard distribution Modules collection.

Adding this block near the top of wechat_qrcode/CMakeLists.txt succeeded for me:

# iconv support isn't automatic on some systems
if(CMAKE_VERSION VERSION_GREATER 3.11)
  find_package(Iconv QUIET)
  if(Iconv_FOUND)
    ocv_target_link_libraries(${the_module} Iconv::Iconv)
  endif()
endif()

It's also safe on POSIX, I built that tree under Fedora and you just see this go by in the CMake output:

-- Performing Test Iconv_IS_BUILT_IN
-- Performing Test Iconv_IS_BUILT_IN - Success
-- wechat_qrcode: Download: detect.caffemodel
-- wechat_qrcode: Download: detect.prototxt
-- wechat_qrcode: Download: sr.caffemodel
-- wechat_qrcode: Download: sr.prototxt

...Can you comment on why you settled on disabling iconv instead, @berak ?

[berak]

@ferdnyc ofc, linking iconv properly is much better than disabling it ;)

see previous discussion here

(tbh, i stopped fiddling with cmake, when dddgz proposed disabling it)

[ferdnyc]

@berak Perfect, that's the answer I was hoping for! I'll go ahead and open a new PR to link libiconv in to wechat_qrcode, and let the downstream MSYS2 packaging maintainers know that the update I already submitted is the way forward, then. Thanks!