[macOS] Build: Will not compile if there is a space in the containing directory

Building from, for example, `Downloads/maxcso/` folder will work fine. 

If I build from `Downloads/Test Folder/maxcso/` the build will fail with: 
```
make: *** No rule to make target `Folder/maxcso/Folder/maxcso/zopfli/src/zopfli/blocksplitter.o', needed by `maxcso'.  Stop.
```
