My motivation is to experiment quirc on macOS, where
neither of SDL1 or V4L2 work.
```
% brew install opencv
% brew install pkg-config
% brew install bsdmake
% bsdmake quirc-demo-opencv
% ./quirc-demo-opencv
```
Note: the macOS version of GNU make (/usr/bin/make) is a bit old
for quirc's Makefile.