configure: fail configuration with missing lex/flex (#3345)

Closes #3343

configure

@@ -6484,7 +6484,9 @@ fi
 rm -f conftest.l $LEX_OUTPUT_ROOT.c
 
 fi
-if test "$LEX" = "lex"; then
+if test "x$LEX" = "x:"; then
+    as_fn_error $? "*** Unable to locate lex." "$LINENO" 5
+elif test "$LEX" = "lex"; then
     # Extract the first word of "lex", so it can be a program name with args.
 set dummy lex; ac_word=$2
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
@@ -6528,7 +6530,7 @@ fi
 
 
     if test "$LEXPATH" = "no"; then
-	as_fn_error $? "*** Unable to locate lex." "$LINENO" 5
+        as_fn_error $? "*** Unable to locate lex." "$LINENO" 5
     fi
 fi
 

configure.ac

@@ -471,10 +471,12 @@ AC_SUBST(LIBRARY_DIRS)
 AC_PROG_INSTALL
 
 AC_PROG_LEX(noyywrap)
-if test "$LEX" = "lex"; then
+if test "x$LEX" = "x:"; then
+    AC_MSG_ERROR([*** Unable to locate lex.])
+elif test "$LEX" = "lex"; then
     AC_PATH_PROG(LEXPATH, lex, no)
     if test "$LEXPATH" = "no"; then
-	AC_MSG_ERROR([*** Unable to locate lex.])
+        AC_MSG_ERROR([*** Unable to locate lex.])
     fi
 fi