include zlib lib dir in link path dirs if it is loaded

This is required for building WPS 3.5 on SL6 with zlib 1.2.7 as an (indirect) dependency, to avoid problems like:

```
/apps/gent/SL6/magnycours/software/libpng/1.6.3-ictce-4.1.13/lib/libpng.so: undefined reference to `inflateReset2@ZLIB_1.2.3.4'
make[1]: [ungrib.exe] Error 1 (ignored)
```