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

Makefile

@@ -234,6 +234,10 @@ ifeq ($(OSARCH),OpenBSD)
   _ASTCFLAGS+=-pthread -ftrampolines
 endif
 
+ifeq ($(OSARCH),linux-uclibc)
+  AST_LIBS+=-lpthread -ldl
+endif
+
 ifeq ($(OSARCH),SunOS)
   _ASTCFLAGS+=-Wcast-align -DSOLARIS -I../include/solaris-compat -I/opt/ssl/include -I/usr/local/ssl/include -D_XPG4_2 -D__EXTENSIONS__
 endif