Can't reproduce with GCC 9.2.1 (Fedora 31).

Please add more information about your build environment, dump exact compiler message and related compiler's build options (through make VERBOSE or ninja -v).

With: GCC 9.2.0, cmake 3.15.2
```
cmake opencv
-DCMAKE_BUILD_TYPE=Release
-DCMAKE_INSTALL_PREFIX=/usr/local
-DCMAKE_SKIP_INSTALL_RPATH:BOOL=ON -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -DOPENCV_ENABLE_NONFREE=ON -DWITH_JASPER=OFF -DWITH_TBB=ON -DWITH_IPP=ON -DWITH_ITT=OFF -DWITH_CUDA=OFF -DWITH_OPENCL=OFF -DWITH_OPENCL_SVM=OFF -DWITH_OPENCLAMDFFT=OFF -DWITH_OPENCLAMDBLAS=OFF -DWITH_LAPACK=OFF -DWITH_FFMPEG=ON -DBUILD_TESTS=OFF -DBUILD_PERF_TESTS=OFF -DBUILD_opencv_xfeatures2d=ON -DPYTHON2_CVPY_SUFFIX=.so -DPYTHON3_CVPY_SUFFIX=.so -DOPENCV_EXTRA_MODULES_PATH=opencv_contrib-3.4.5/modules -DWITH_PROTOBUF=ON -DBUILD_PROTOBUF=OFF -DPROTOBUF_UPDATE_FILES=ON -DENABLE_CXX11=ON -DBUILD_SHARED_LIBS=OFF
-DENABLE_PIC=ON
-DBUILD_FAT_JAVA_LIB=ON

[ 11%] Generating precomp.hpp.gch/opencv_core_Release.gch
cd opencv/modules/core && cmake -E make_directory opencv/modules/core/precomp.hpp.gch
cd opencv/modules/core &&
g++ -O3 -DNDEBUG "-D__OPENCV_BUILD=1" "-D_USE_MATH_DEFINES" "-D__STDC_CONSTANT_MACROS" "-D__STDC_LIMIT_MACROS" "-D__STDC_FORMAT_MACROS" "-DCVAPI_EXPORTS" -std=c++11
-I"opencv/3rdparty/ippicv/ippicv_lnx/icv/include" -I"opencv/3rdparty/ippicv/ippicv_lnx/iw/include" -I"opencv/3.4.5/modules/core/src" -I"opencv/modules/core/include" -I"opencv/modules/core"
-O3 -g -pipe -Wall -Wno-error -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector --param=ssp-buffer-size=4 -fno-omit-frame-pointer -momit-leaf-frame-pointer
-fno-reorder-blocks-and-partition -Wl,-z,relro -march=corei7 -mtune=skylake -nostdinc -grecord-gcc-switches
-fPIC -fpermissive -fno-omit-frame-pointer -fsigned-char
-W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations
-Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wno-narrowing -Wno-delete-non-virtual-dtor -Wno-comment
-Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -Wno-long-long
-pthread -fomit-frame-pointer -ffunction-sections -fdata-sections -msse -msse2 -msse3 -fvisibility=hidden -fvisibility-inlines-hidden -fPIC
-x c++-header -o opencv/modules/core/precomp.hpp.gch/opencv_core_Release.gch opencv/modules/core/precomp.hpp
ld: undefined reference to `main'
collect2: error: ld returned 1 exit status

Thank you for update!

Looks like, linker flag "-Wl,-z,relro" confuses GCC. It is not GCC 9 specific, there is output of GCC 8.3.1:

/usr/bin/ld: /usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o: in function `_start':
(.text+0x24): undefined reference to `main'
collect2: error: ld returned 1 exit status

"-c" fixes problem again.

As a bugfix this patch should go into 3.4 branch first. We will merge changes from 3.4 into master regularly (weekly/bi-weekly).

So, please:

• change "base" branch of this PR: master => 3.4 (use "Edit" button near PR title)
• rebase your commits from master onto 3.4 branch. For example:
  git rebase -i --onto upstream/3.4 upstream/master
  (check list of your commits, save and quit (Esc + "wq" + Enter)
  where upstream is configured by following this GitHub guide and fetched (git fetch upstream).
• push rebased commits into source branch of your fork (with --force option)

Note: no needs to re-open PR, apply changes "inplace".