configure: don't require Flex

Flex should not be required to build Bison or run the test suite (of
course it is needed for maintaining Bison).  Yet the Automake
conditional FLEX_WORKS does not work.

* m4/flex.m4 (_AC_PROG_LEX_YYTEXT_DECL): Since this is called
conditionally, don't define LEX_IS_FLEX here, but rather...
(AC_PROG_LEX): here.
* configure.ac: Be more cautious about possibly undefined variables.

configure.ac

@@ -211,10 +211,10 @@ AC_CONFIG_FILES([src/yacc], [chmod +x src/yacc])
 # Checks for programs.
 AM_MISSING_PROG([DOT], [dot])
 AC_PROG_LEX
-$LEX_IS_FLEX || test "X$LEX" = X: || {
+if ! "$LEX_IS_FLEX" || test "X$LEX" = X:; then
   AC_MSG_WARN([bypassing lex because flex is required])
   LEX=:
-}
+fi
 AM_CONDITIONAL([FLEX_WORKS], [$LEX_IS_FLEX])
 AM_CONDITIONAL([FLEX_CXX_WORKS],
   [$LEX_IS_FLEX && test $bison_cv_cxx_works = yes])

m4/flex.m4

@@ -15,7 +15,10 @@ AC_DEFUN_ONCE([AC_PROG_LEX],
 [AC_CHECK_PROGS([LEX], [flex lex], [:])
 if test "x$LEX" != "x:"; then
   _AC_PROG_LEX_YYTEXT_DECL
-fi])
+fi
+AC_SUBST([LEX_IS_FLEX],
+         [`test "$ac_cv_prog_lex_is_flex" = yes && echo true || echo false`])dnl
+])
 
 
 # _AC_PROG_LEX_YYTEXT_DECL
@@ -37,8 +40,6 @@ else
   ac_cv_prog_lex_is_flex=no
 fi
 ])
-AC_SUBST([LEX_IS_FLEX],
-         [`test "$ac_cv_prog_lex_is_flex" = yes && echo true || echo false`])dnl
 
 cat >conftest.l <<_ACEOF[
 %%