The line
OPENCV = pkg-config --cflags --libs opencv4
doesn't evaluate the command, for that you need to wrap with
$(shell ...)
. Also, you need to tell the compiler for each module where to find the include files for opencv. I think the following should do the trick.
CFLAGS = -std=c++11 $(shell pkg-config --cflags opencv4)
LIBS = $(shell pkg-config --libs opencv4)