build: enable building with uClibc

This patch has been included in Gentoo distribution for at least since
asterisk 1.8, but there are references in the logs going back as far as
1.0.0 - not sure if this is still required in any way, it does apply,
and it doesn't (as far as we can determine) cause build failures.

Change-Id: I46d8845e30200205e80580680bf060aa3012ba54