Without it, when I run 

`brew uninstall fontforge; brew install fontforge --HEAD --debug --with-x`

then it fails. I press 5 to get a shell, and press up to get the previous command and rerun it, and this is what I get:

```
./configure --prefix=/usr/local/Cellar/fontforge/HEAD --enable-double --without-freetype-bytecode --without-cairo --without-pango --with-static-imagelibs --enable-pyextension
...
checking for library containing XOpenDevice... -lXi
checking for library containing XkbQueryExtension... none required
checking for ZLIB... no
configure: error: Package requirements (zlib) were not met:

No package 'zlib' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables ZLIB_CFLAGS
and ZLIB_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.
bash-3.2$ 
```