* configure.ac: Use AC_CHECK_DECLS instead of gcc_AC_CHECK_DECLS

* configure.ac: Use AC_CHECK_DECLS instead of gcc_AC_CHECK_DECLS
and BFD_NEED_DECLARATION.  Don't check if a declaration of strdup
is necessary.
* configure, config.in: Regenerate.
* utils.c: Adjust for usage of AC_CHECK_DECLS instead of
BFD_NEED_DECLARATION.  Never provide a prototype for
caninicalize_file_name.
* gdb_string.h: Adjust for usage of AC_CHECK_DECLS instead of
BFD_NEED_DECLARATION.