I am using pkg-config 0.26 on OS X Lion.
When I echo $PKG_CONFIG_PATH, it seems that the variable is blank.
When I assign into it, e.g. PKG_CONFIG_PATH=/path/to/my/folder/containing/pc/files, and then run pkg-config --list-all, the .pc files in the directory that I ostensibly just added are not found.
I have been side-stepping this issue by soft linking to individual .pc files from /opt/local/share/pkgconfig, but I would prefer to figure out what the right way is.