Make code compile when basename() is POSIX compliant.

In addition to the previous change I made to ar.c, pull in another
basename() related fix. This change is similar to the one made to the
ELF Toolchain version of ar, with the difference that the ELF Toolchain
version lacks error handling for the strdup() call.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D6467