Only include libgen.h if we have a basename as well.

Mainly meant to deal with the IRIX case which seems to requrie a "-lgen" lib
to find the basename function and thus without the gen lib, it finds the
header but not the function and our replacement function has a prototype
that doesn't match the IRIX one.

A different approach would be to make configure detect and use -lgen for the
systems that require it.