Here's a typical scenario - you install software from source, spend an hour figuring out the configure options, and finally run:
./configure --prefix=/a/b/c --with-X --with-Y=/usr/local/Y/lib \
--disable-Z --with-extension=foo --build=bar ...
A year passes by, a new software version has come out, and you want to upgrade. To do that you need to run configure again on the new source code. However you've forgotten what the configure flags were, but you still do have the old source code and the old build somewhere in your home directory.
How to recover the configure string a year later when you've forgotten what the original configure string was?