busybox

Makefile: add missing libraries to LDLIBS

Static build with CONFIG_PAM fails on Debian Jessie due to undefined
references to libdl and libaudit.

Static build without pam, but with libcrypt required fails as well due
to undefined references to libpthread.

Fix these two cases by adding the missing libraries to LDLIBS when
appropriate.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>