Build with highgui and videoio modules

[kalwalt]

I'm trying to build with highgui and videoio modules, in this branch https://github.com/webarkit/opencv-em/tree/with-highgui. But the process stops at videoio compilation, see log:

emcc:WARNING: --llvm-lto ignored when using llvm backend
../modules/videoio/src/backend_plugin.cpp:76:30: warning: unused parameter 'h' [-Wunused-parameter]
void* getSymbol_(LibHandle_t h, const char* symbolName)
                             ^
../modules/videoio/src/backend_plugin.cpp:76:45: warning: unused parameter 'symbolName' [-Wunused-parameter]
void* getSymbol_(LibHandle_t h, const char* symbolName)
                                            ^
../modules/videoio/src/backend_plugin.cpp:83:1: error: non-void function does not return a value [-Werror,-Wreturn-type]
}
^
../modules/videoio/src/backend_plugin.cpp:86:50: warning: unused parameter 'filename' [-Wunused-parameter]
LibHandle_t libraryLoad_(const FileSystemPath_t& filename)
                                                 ^
../modules/videoio/src/backend_plugin.cpp:97:1: error: non-void function does not return a value [-Werror,-Wreturn-type]
}
^
../modules/videoio/src/backend_plugin.cpp:100:34: warning: unused parameter 'h' [-Wunused-parameter]
void libraryRelease_(LibHandle_t h)
                                 ^
4 warnings and 2 errors generated.

@ThorstenBux it was possible with opencv 4.1 but maybe is not possible with 4.5.0?

[kalwalt]

closing because we don't need anymore this