Enaable mostly-static build on OSX.
Enaable mostly-static build on OSX.

```bash
$ otool -L bin/*
bin/cjxl:
	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 904.4.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1292.60.1)
bin/djxl:
	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 904.4.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1292.60.1)
```

libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl