Without this change, I’m getting the following error:

```
cc  blocksplitter.c cache.c deflate.c gzip_container.c hash.c katajainen.c lz77.c squeeze.c tree.c util.c zlib_container.c zopfli_lib.c woff.c sfnt2woff.c -o sfnt2woff-zopfli -lz
/tmp/ccMWhalm.o: In function `ZopfliCalculateEntropy':
tree.c:(.text+0x290): undefined reference to `log'
tree.c:(.text+0x2c7): undefined reference to `log'
tree.c:(.text+0x375): undefined reference to `log'
collect2: error: ld returned 1 exit status
Makefile:6: recipe for target 'sfnt2woff-zopfli' failed
make: *** [sfnt2woff-zopfli] Error 1
```

The manpage `log(3)` mentions “Link with -lm.”, which this change does.