(handle_dependencies): Set $require_file_found{'depcomp'}

(handle_dependencies): Set $require_file_found{'depcomp'}
if the depcomp file exists, before calling require_config_file on
`depcomp'.  This makes require_file_internal skip its buggy existence
test that would make automake fail (with `required file `lib/depcomp'
not found') when AC_CONFIG_AUX_DIR is not set.

This change change does not fix the existing bug where, if `depcomp'
is not present in the build directory (at the top level), automake
still gives the inaccurate diagnostic about *lib*/depcomp rather
than ./depcomp.