Support toolchain and custom arguments for CMake

[SageMik]

Hello, I'am cross-compiling OpenCV to Android on Windows and using OpenCV_DIR to do it by CMake. But I got this error:

[opencv 0.95.1] === cmake find-package link probe command: "cmake" "-S" "xxx\\opencv-0.95.1\\cmake" "-DOCVRS_PACKAGE_NAME=OpenCV" "-DCMAKE_BUILD_TYPE=Debug" "--find-package" "-DCOMPILER_ID=GNU" "-DLANGUAGE=CXX" "-DMODE=LINK" "-DNAME=OpenCV"

...

  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: ld.lld: error: unable to find library -lkernel32␍
          ld.lld: error: unable to find library -luser32␍
          ld.lld: error: unable to find library -lgdi32␍
          ld.lld: error: unable to find library -lwinspool␍
          ld.lld: error: unable to find library -lshell32␍
          ld.lld: error: unable to find library -lole32␍
          ld.lld: error: unable to find library -loleaut32␍
          ld.lld: error: unable to find library -luuid␍
          ld.lld: error: unable to find library -lcomdlg32␍
          ld.lld: error: unable to find library -ladvapi32␍
          clang: error: linker command failed with exit code 1 (use -v to see invocation)␍
          ␍

I found the reason lies here. It doesn't provide any toolchain so CMake --find-package ouputs linker arguments binding Windows libraries, which are unavailable on Android.

opencv-rust/build/library.rs

Lines 616 to 629 in 5ee7bc6

	fn run_probe(
	&self,
	include_paths: Option<EnvList>,
	link_paths: Option<EnvList>,
	link_libs: Option<EnvList>,
	) -> Result<Library> {
	match self {
	Probe::Environment => Library::probe_from_paths(include_paths, link_paths, link_libs),
	Probe::PkgConfig => Library::probe_pkg_config(include_paths, link_paths, link_libs),
	Probe::CMake => Library::probe_cmake(include_paths, link_paths, link_libs, None, None, None),
	Probe::VcpkgCMake => Library::probe_vcpkg_cmake(include_paths, link_paths, link_libs),
	Probe::Vcpkg => Library::probe_vcpkg(include_paths, link_paths, link_libs),
	}
	}

I tried Android NDK CMake Toolchain -DCMAKE_TOOLCHAIN_FILE=$NDK/build/cmake/android.toolchain.cmake and added -DOpenCV_DIR=path/to/opencv after finding that OpenCV_DIR environment variable is not working when setting toolchain (using cmake -LA xxx can check which variables are working). Finally I succeed to cross-compile it.

So could you please provide environment variables like OPENCV_CMAKE_TOOLCHAIN_FILE and OPENCV_CMAKE_ARGS, so we can pass arguments to CMake to make cross-compilation successful?

[twistedfall]

Thank you for the investigation! I might be a bit out of the water on this particular setup of CMake, do you think it's possible for you to make a PR with the proposed changes? I can then possibly adjust it to fit the project better, but to at least get the general idea right