More NetBSD patches from Johnny Lam.

More NetBSD patches from Johnny Lam.

Remove the need for a $prefix setting that was added in the
previous patch I sent.	Also add the proper rpath options
to the linker flags depending on whether we're on a ELF or
a.out platform.	 These were tested both in a non-pkgsrc and
pkgsrc build.

Append to $libs instead of overwriting its value.  This was
causing $(LDLOADLIBS) to not get added to the dynamic_libs
link command on NetBSD systems and was the cause of the GDBM
problems reported.  Also use '$(LDLOADLIBS)' instead of
expanding its value so that it is more easily overridden in
the Makefile by "make all LDLOADLIBS=...".

Remove "-L/usr/local/lib" because the location of libgdbm.so
is already added during the Configure process, and this
spurious addition may cause the wrong libgdbm.so to be linked
against as a result.

p4raw-id: //depot/perl@16233