the configure option for mandir doesn't work. Regardless of what is used the mandir used by make will always be PREFIX/man. I even hardcoded the path I wanted to use in the configure file where the mandir variable is initialized and it still used PREFIX/man. This ends up with an error during make install:
install: PREFIX/man/man1/cjpeg.1: No such file or directory
/usr/bin/install -c -m 644 ./cjpeg.1 PREFIX/man/man1/cjpeg.1
Try installing it anywhere other than /usr/local and it will fail at this point.