libvirt_proxy: link with -lpthread if needed

Continuation of earlier patches to fix LIB_PTHREAD, only
triggered by ./configure --with-xen-proxy (a la autobuild.sh).

* proxy/Makefile.am (libvirt_proxy_LDADD): Add LIB_PTHREAD.

proxy/Makefile.am

@@ -34,7 +34,7 @@ libvirt_proxy_SOURCES = libvirt_proxy.c \
             @top_srcdir@/src/xen/xs_internal.c
 libvirt_proxy_LDFLAGS = $(WARN_CFLAGS) $(XEN_LIBS)
 libvirt_proxy_DEPENDENCIES =
-libvirt_proxy_LDADD = ../gnulib/lib/libgnu.la
+libvirt_proxy_LDADD = ../gnulib/lib/libgnu.la $(LIB_PTHREAD)
 
 install-exec-hook:
 	chmod u+s $(DESTDIR)$(libexecdir)/libvirt_proxy