bug#9859: automake bug#9859 has been fixed in Automake-NG
JFTR: this bug has been fixed in the Automake-NG fork of Automake
(which assumes GNU make is used to execute its generated makefiles);
see commits v1.11-1838-gdc04691 "[ng] yacc, lex: fix subdir VPATH
builds" and v1.11-1839-ge5b964c "[ng] yacc, lex, compiling: better
use of '$<' (simplify and fix bug)" in the 'ng/master' branch of
the official automake repository.