The issue is found in master, introduced by PR #526
Failure log:

In file included from /usr/local/include/opencv4/opencv2/core/core_c.h:48:0,
from libavfilter/vf_ocv_overlay.c:32:
/usr/local/include/opencv4/opencv2/core/types_c.h:367:20: warning: function declaration isn't a prototype [-Wstrict-prototypes]
CV_INLINE IplImage cvIplImage()
^~~~~~~~~~
/usr/local/include/opencv4/opencv2/core/types_c.h:717:19: warning: function declaration isn't a prototype [-Wstrict-prototypes]
CV_INLINE CvMatND cvMatND()
^~~~~~~
libavfilter/vf_ocv_overlay.c:40:10: fatal error: json-c/json.h: No such file or directory
#include <json-c/json.h>
^~~~~~~~~~~~~~~
compilation terminated.
make: *** [libavfilter/vf_ocv_overlay.o] Error 1
ffbuild/common.mak:59: recipe for target 'libavfilter/vf_ocv_overlay.o' failed
make: *** Waiting for unfinished jobs....
CC libavfilter/vf_owdenoise.o
The command '/bin/bash -o pipefail -c cd /home/FFmpeg && export PKG_CONFIG_PATH="/usr/local/lib/x86_64-linux-gnu/pkgconfig" && ./configure --prefix="/openvino/deployment_tools/inference_engine/lib/intel64 " --libdir=/usr/local/lib/x86_64-linux-gnu --extra-libs="-lpthread -lm" --enable-shared --enable-gpl bvpx --enable-libx264 --enable-libx265 --enable-libdav1d --enable-libinference_engine_c_api --enable-librdkafka --enable-libopencv && make -j8 && ma
VCAC-A/ubuntu-18.04/analytics/dev/CMakeFiles/build_vcaca_analytics_devel_ubuntu1804.dir/build.make:57: recipe for target 'VCAC-A/ubuntu-18.04/analytics/dev/C
make[2]: *** [VCAC-A/ubuntu-18.04/analytics/dev/CMakeFiles/build_vcaca_analytics_devel_ubuntu1804] Error 2
CMakeFiles/Makefile2:26978: recipe for target 'VCAC-A/ubuntu-18.04/analytics/dev/CMakeFiles/build_vcaca_analytics_devel_ubuntu1804.dir/all' failed
make[1]: *** [VCAC-A/ubuntu-18.04/analytics/dev/CMakeFiles/build_vcaca_analytics_devel_ubuntu1804.dir/all] Error 2
Makefile:94: recipe for target 'all' failed
make: *** [all] Error 2