I did install zlib. It got installed as part of something else, but I installed it by hand just in case after I saw the error. Here is my console output so you can see what's going on (including me checking that zlib is installed):
13:45:25 [scott@mikuru:~/opensource/fceu2.1.5] $ sudo port install zlib
---> Cleaning zlib
---> Scanning binaries for linking errors: 100.0%
---> No broken files found.
13:45:29 [scott@mikuru:~/opensource/fceu2.1.5] $ CFLAGS=-I/opt/local/include LDFLAGS=-L/opt/local/lib scons
scons: Reading SConscript files ...
platform: darwin
Checking for inflate in C library z... no
AssertionError: please install: zlib:
File "/Users/scott/opensource/fceu2.1.5/SConstruct", line 58:
assert conf.CheckLibWithHeader('z', 'zlib.h', 'C', 'inflate;', 1), "please install: zlib"
I checked the SConfig script, and it looks like it's just looking for the headers and hasn't even gotten to the point of compiling yet. :\