Just a side note to add to the post by tomdkat, which will work fine:
You can usually specify the PKG_CONFIG_PATH variable on the command line, and pass it to the configure script. I've never had a package that didn't have the *.pc file in either /usr/lib/pkgconfig or /usr/local/lib/pkgconfig, although that's certainly not a guarantee. Anyway, if the needed file is in one of the two spots I mentioned, then most software will be fine with
which will search for the package in those directories, in said order.
Or you can set the environment variable as stated.