Bug 786638 updated the manual and included this:
+By default, dynamic tests are linked to run with the installed C library.
+This option hardcodes the newly built C library path in dynamic tests
+so that they can be invoked directly.
However, the glibc build system does not actually support this option.
Platform Tools glibc has reviewed this bug and decided to mark this CLOSED WONTFIX. The issue does not directly impact any customers.