ERROR: opencv not found using pkg-config

[rustythehermit]

Summary of the bug:
How to reproduce:

> pkg-config opencv --libs --cflags
-I/usr/include/opencv4 -lopencv_highgui -lopencv_ml -lopencv_objdetect -lopencv_photo -lopencv_stitching -lopencv_video -lopencv_calib3d -lopencv_features2d -lopencv_dnn -lopencv_flann -lopencv_videoio -lopencv_imgcodecs -lopencv_imgproc -lopencv_core

> pkg-config opencv4 --libs --cflags
-I/usr/include/opencv4 -lopencv_highgui -lopencv_ml -lopencv_objdetect -lopencv_photo -lopencv_stitching -lopencv_video -lopencv_calib3d -lopencv_features2d -lopencv_dnn -lopencv_flann -lopencv_videoio -lopencv_imgcodecs -lopencv_imgproc -lopencv_core


> set -x ldflags -Wl,-rpath=/usr/local/lib -flto=auto -fuse-linker-plugin

> ./configure \
--prefix=/usr/local \
--extra-ldflags="-L/usr/local/lib -L/usr/local/lib64 $ldflags" \
--extra-libs="-lpthread -lm" \
--ar=gcc-ar --disable-debug --disable-doc \
--disable-static --enable-shared --enable-pthreads \
--enable-hardcoded-tables --arch=x86_64 --enable-x86asm \
--enable-avcodec --enable-bzlib --enable-gcrypt --enable-gmp \
--enable-gnutls --enable-fontconfig --enable-lcms2 --enable-libaom \
--enable-libass --enable-libdav1d --enable-libdrm --enable-libfdk-aac \
--enable-libfreetype --enable-libfribidi --enable-libmfx --enable-libmp3lame \
--enable-libopus --enable-libpulse --enable-librsvg --enable-libspeex \
--enable-libsvtav1 --enable-libtheora --enable-libvidstab \
--enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp \
--enable-libx264 --enable-libx265 --enable-vulkan --enable-libshaderc \
--enable-libxml2 --enable-libxvid --enable-libtensorflow \
--enable-opengl --enable-pixelutils --enable-postproc --enable-rdft \
--enable-sdl2 --enable-vaapi --enable-libbs2b --enable-libopencv \
--enable-zlib --enable-ffplay --enable-gpl --enable-nonfree \
--enable-version3 --enable-lto --enable-libsnappy \
--enable-opencl --enable-openal --disable-avx512 --enable-libzimg \
--enable-libjxl --enable-libplacebo --enable-vapoursynth \
--enable-librubberband --enable-libopenjpeg --enable-libtesseract \
--enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libsoxr \
--disable-libxcb --disable-libxcb-shm --disable-libxcb-shape --disable-libxcb-xfixes  \
--disable-vdpau --disable-cuda-llvm || exit $status

ERROR: opencv not found using pkg-config

If you think configure made a mistake, make sure you are using the latest
version from Git.  If the latest version fails, report the problem to the
ffmpeg-user@ffmpeg.org mailing list or IRC #ffmpeg on irc.libera.chat.
Include the log file "ffbuild/config.log" produced by configure as this will help
solve the problem.

Patches should be submitted to the ffmpeg-devel mailing list and not this bug tracker.

[Cosmin Stejerean]

Generally when something like this fails the interesting output is in ffbuild/config.log, the error message there should show why the specific check for opencv failed.