Fails to build on macOS, i think the included maus library doesn't quite work with it

```
% ./configure 
Configuration done
mkdir -p build
cc -std=c99 -Wall -Wextra -O2  -Iinclude -c source/maus.c -o build/maus.o
In file included from source/maus.c:12:
include/maus.h:107:2: error: unknown type name 'MausBackend'
  107 |         MausBackend    backend;
      |         ^
1 error generated.
make: *** [build/maus.o] Error 1
```