opcodes: Fix RPATH not being set for dynamic libbfd dependency

opcodes: Fix RPATH not being set for dynamic libbfd dependency

If built as a shared library, libopcodes has a load-time dependency on
libbfd, which is recorded in the dynamic section, however without a
corresponding RPATH entry for the directory to find libbfd in.  This
causes loading to fail whenever libbfd is only pulled by libopcodes
indirectly and libbfd has been installed in a directory that is not in
the dynamic loader's search path.

It does not happen with the programs included with binutils or GDB,
because they all also pull libbfd when using libopcodes, but it can
happen with external software, e.g.:

$ gdbserver --help
gdbserver: error while loading shared libraries: libbfd-[...].so: cannot open shared object file: No such file or directory
$

(not our `gdbserver').

Indirect dynamic dependencies are handled by libtool automatically by
adding RPATH entries as required, however our setup for libopcodes
prevents this from happening by linking in libbfd with an explicit file
reference sneaked through to the linker directly behind libtool's back
via the `-Wl' linker command-line option rather than via `-l' combined
with a suitable library search path specified via `-L', as it would be
usually the case, or just referring to the relevant .la file in a fully
libtool-enabled configuration such as ours.

According to an observation in the discussion back in 2007[1][2][3] that
has led to the current arrangement it is to prevent libtool from picking
up the wrong version of libbfd.  It does not appear to be needed though,
not at least with our current libtool incarnation, as directly referring
`libbfd.la' does exactly what it should, as previously suggested[4], and
with no link-time reference to the installation directory other than to
set RPATH.  Uninstalled version of libopcodes has libbfd's build-time
location prepended to RPATH too, as also expected.

Use a direct reference to `libbfd.la' then, making the load error quoted
above go away.  Alternatively `-L' and `-l' could be used to the same
effect, but it seems an unnecessary complication and just another way to
circumvent rather than making use of libtool.

References:

[1] "compile failure due to undefined symbol",
    <https://sourceware.org/ml/binutils/2007-08/msg00476.html>

[2] same, <https://sourceware.org/ml/binutils/2007-09/msg00000.html>

[3] same, <https://sourceware.org/ml/binutils/2007-10/msg00019.html>

[4] same, <https://sourceware.org/ml/binutils/2007-10/msg00034.html>

	opcodes/
	* Makefile.am: Remove obsolete comment.
	* configure.ac: Refer `libbfd.la' to link shared BFD library
	except for Cygwin.
	* Makefile.in: Regenerate.
	* configure: Regenerate.