tests: truly use Lex and Yacc selected by configure

tests: truly use Lex and Yacc selected by configure

When 'yacc-work' was merged into 'yl-work-for-master' in commit
v1.11-1910-ge6c40d4 (12-02-2012), the values for $YACC and $LEX
stopped being substituted directly by config.status, and started
being substituted by a Makefile rule using the contents of the
$(YACC) and $(LEX) makefile variables.

Unfortunately, our configure.ac was calling AM_SUBST_NOTMAKE on
'YACC' and 'LEX' (see commit v1.11-284-g34ca92b of 29-01-2011,
"tests: don't define YACC and LEX in the Makefiles", for a
rationale), so that the $(YACC) and $(LEX) in the Makefile simply
hold the default make-provided values (that is, "yacc" and "lex"
respectively), independently of what yacc and lex programs were
defined at configure time.

Apart from reducing coverage and creating confusion, this behaviour
also caused spurious failures on systems lacking a yacc and/or lex
programs.

Problem reported by Peter Rosin.  References:
 <http://lists.gnu.org/archive/html/automake-patches/2012-02/msg00131.html>
 <http://lists.gnu.org/archive/html/automake-patches/2012-02/msg00132.html>

* configure.ac (AM_SUBST_NOTMAKE): Don't call anymore on 'YACC' nor
on 'LEX'.