control linking against zlib, just as we added ZOPFLI_LIBS and ZOPFLI_CFLAGS
for the Zopfli library.

build with an external Zopfli library. Set their default values such that the
bundled Zopfli library is still used by default. Re-order the linker flags so
that both cases work as expected.

Fix linker flag order

Separating source files by origin (and license) makes it easy for users who
want to build with an external zopfli library to remove the bundled sources,
ensuring they are not used. Unbundling libraries is mandatory for packaging in
many popular Linux distributions, and can be useful for other applications as
well. Additional changes will be needed to make it easy to select an external
zopfli library.

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.