To build nginx on Mac OS X Snow Leopard, I use the following options to configure
./configure --with-ld-opt="-L/usr/local/lib" --with-cc-opt="-I/usr/local/include"
If the ld and cc options are not given, it results in an error reported in a ticket on the nginx trac.
Notice that the is no space between -L and /usr/local/lib and likewise for -I and /usr/local/include. This is different from what is written in the FreeBSD section of the InstallNotes on the nginx website.