At work I maintain a handful of custom PHP extensions.  When someone reports a problem with one of the extensions, I want to fire up gdb right away and see exactly what is going on.  In order to do this, I build a custom php binary with debugging enabled.  I leave this binary inside my home directory so as not to affect my installed production php binary.  I should now be able to rebuild my custom extensions now with debugging enabled and start debugging.  But wait, the configure script rejects the flag.