System Information

OpenCV python version: 4.10.0
Operating System / Platform: Ubuntu 20.04
Python version: 3.8.10

Detailed description

OpenCV 4.10 fails to build on Ubuntu 20.04 with Freetype enabled in the GAPI module. The build system fails to find ft2build.h, which is located in /usr/include/freetype2. This path is discovered in the call to ocv_check_modules in init.cmake, but the directory "/usr/include" is added to the end of the list of paths. This causes the gcc6 bug workaround in ocv_target_include_directories to be hit because the string representation of the array containing the Freetype2 include paths ends in ";/usr/include". This issue seems to affect other libraries that are discovered within the build system that use ocv_check_modules as well. I have a branch that I will attach that resolves this issue by moving the check for "/usr/include" to within the loop that iterates over the directories to include.

  In file included from /tmp/pip-req-build-ifckk0ry/opencv/modules/gapi/src/backends/render/ft_render.cpp:12:
  /tmp/pip-req-build-ifckk0ry/opencv/modules/gapi/src/backends/render/ft_render_priv.hpp:15:10: fatal error: ft2build.h: No such file or directory
     15 | #include <ft2build.h>
        |          ^~~~~~~~~~~~
  compilation terminated.
  make[2]: *** [modules/gapi/CMakeFiles/opencv_gapi.dir/build.make:1196: modules/gapi/CMakeFiles/opencv_gapi.dir/src/backends/render/ft_render.cpp.o] Error 1
  make[2]: *** Waiting for unfinished jobs....
  make[2]: Leaving directory '/tmp/pip-req-build-ifckk0ry/_skbuild/linux-x86_64-3.8/cmake-build'
  make[1]: *** [CMakeFiles/Makefile2:5547: modules/gapi/CMakeFiles/opencv_gapi.dir/all] Error 2
  make[1]: *** Waiting for unfinished jobs....

Steps to reproduce

attempt to build OpenCV 4.x on Ubuntu 20.04 with Freetype installed

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues, forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files (videos, images, onnx, etc)