Invalid use of $zlibdir within GDB_AC_CHECK_BFD

Hi,

 The use of $zlibdir when setting LDFLAGS within GDB_AC_CHECK_BFD in
gdb/acinclude.m4 seems to be incorrect unless gdb is configured `--with-system
-zlib'. At configure time it becomes (see config/zlib.m4)

-L$(top_builddir)/../zlib

with $(top_builddir) being a make(!) variable. This makes a number of
gdb/configure
tests fail unless a user has got an installed system zlib for the host
of GDB. As a result,
one may quite unexpectedly obtain GDB without ELF support.