examples: don't run those that require f?lex when it's not available

Reported by Bruno Haible.
http://lists.gnu.org/archive/html/bug-bison/2019-05/msg00026.html

* configure.ac (FLEX_WORKS): New.
* examples/c/lexcalc/local.mk, examples/c/reccalc/local.mk: Use it.

configure.ac

@@ -215,6 +215,7 @@ $LEX_IS_FLEX || test "X$LEX" = X: || {
   AC_MSG_WARN([bypassing lex because flex is required])
   LEX=:
 }
+AM_CONDITIONAL([FLEX_WORKS], [$LEX_IS_FLEX])
 AM_CONDITIONAL([FLEX_CXX_WORKS],
   [$LEX_IS_FLEX && test $bison_cv_cxx_works = yes])
 AC_PROG_YACC