Karl Berry Olaf Bachmann and many others.
Some packages require, or can optionally use, other software packages which are already installed.
The user can give configure command line options to specify which such external software to use. This behavior permits configuring a source tree containing multiple packages with a top-level configure script when the packages support different options, without spurious error messages about options that some of the packages support. An unfortunate side effect is that option spelling errors are not diagnosed. No better approach to this problem has been suggested so far. Whether each package is used or not by default, and which arguments are valid, is up to you.