Previously we (appropriately, but incorrectly) attempted to depend on
LIBC_NOSSP_PIC and LIBSYS_PIC the main rtld_libc.a. Unfortunately,
variables in dependency lists are expanded at parse time and those
variables are defined in bsd.libnames.mk which *must* be included by
bsd.{lib,prog}.mk. As such, they were undefined and thus expanded to
the empty string resulting in no dependency with predictable and highly
confusing results.
Move the declaration of these dependencies to after the include of
bsd.prog.mk and add comments on both side in hopes of keeping any future
dependencies in sync.