How to build OpenCV

# Configure using CMake
# Add -DOPENCV_EXTRA_MODULES_PATH=<Path to opencv_contrib source>/contrib__ to build opencv_contrib modules
$ cmake .. -G"Unix Makefiles" \
-DCMAKE_TOOLCHAIN_FILE=<Path to toolchain.make> \
-DBUILD_PERF_TESTS=OFF \
-DBUILD_TESTS=OFF \
-DBUILD_ZLIB=ON \
-DCMAKE_BUILD_TYPE=Release \
-DWITH_CUBLAS=OFF \
-DWITH_CUDA=OFF \
-DWITH_CUFFT=OFF \
-DWITH_GSTREAMER=OFF \
-DWITH_GTK=OFF \
-DWITH_MATLAB=OFF \
-DWITH_NVCUVID=OFF \
-DWITH_OPENCL=OFF \
-DWITH_OPENCLAMDBLAS=OFF \
-DWITH_OPENCLAMDFFT=OFF \
-DZLIB_INCLUDE_DIR=<OpenCV source>/3rdparty/zlib

# Compile & install
$ make -j`nproc` install

contents of toolchain.make

set( CMAKE_SYSTEM_NAME Linux )
set( CMAKE_SYSTEM_PROCESSOR armv7-a )
set( CMAKE_C_COMPILER arm-linux-gnueabihf-gcc )
set( CMAKE_CXX_COMPILER arm-linux-gnueabihf-g++ )