At least one gnome application (gthumb) in one of its recent incarnations
(2.14.3) has a configure script that invokes pkg-config(1) in order to check
for the presence of something it refers to only as "zlib", and also to get
set up to use that (e.g. with suitable -L linker options).

Apparently, on Linux systems, there exists a file somewhere called zlib.pc
that can be used by pkg-config to get information about "zlib", however on
FreeBSD it appears that there is a boat load of .pc files, all of them
stored in the directory /usr/local/libdata/pkgconfig, but these only cover
libraries that were installed as part of some port or package, and sadly,
there is no zlib.pc file in there to help  pkg-config figure out how to
properly link with /usr/lib/libz.{a,so} because apparently, on FreeBSD,
that's part of the base system and thus, no zlib.pc file is ever generated
or installed for it.  Bummer.  The result is that the configure step for
gthumb 2.14.3 dies with the following errors:

gnome-config: not found
Package zlib was not found in the pkg-config search path.
Perhaps you should add the directory containing `zlib.pc'
to the PKG_CONFIG_PATH environment variable
No package 'zlib' found

Fix: 

I'm thinking that maybe it would be a Good Thing if the install process for
pkg-config were to create a file called /usr/local/libdata/pkgconfig/zlib.pc

... but what do I know?
How-To-Repeat: 
1)  Fetch source tarball for gthumb-2.14.3.

2)  Unzip & untar.

3)  cd into the main directory

4)  ./configure