extism lib install fails on macOS due to missing /usr/local/include /usr/local/lib folders

Had to `sudo mkdir -p /usr/local/include /usr/local/lib` then `sudo extism lib install` worked:

```
patrick@Mac ~ % ls /usr/local/lib
libextism.a     libextism.dylib pkgconfig
patrick@Mac ~ % ls /usr/local/include
extism.h
```