Do not build libctf for targets that do not use the ELF file format.

Do not build libctf for targets that do not use the ELF file format.

top	* configure.ac (noconfigdirs): Add libctf if the target does not use
	the ELF file format.
	* configure: Regenerate.

binutils* configure.ac (LIBCTF): Export.  Set to empty for non-ELF based
	targets.
	(HAVE_LIBCTF): Define if libctf support is available.
	* Makefile.am (LIBCTF): Set value to @LIBCTF@.
	* objdump.c: Make CTF code conditional upon HAVE_LIBCTF being
	defined.
	* readelf.c: Likewise.
	* configure: Regenerate.
	* Makefile.in: Regenerate.
	* config.in: Regenerate.