fix(scanner): detect yyreject() without %option reject

CHECK_YYREJECT passed the whole matched text, parentheses included, to
all_lower, which is never true for it. A scanner that used yyreject()
had to say %option reject or it failed to compile. The tests all do.

Compare the name instead of the whole text.

src/scan.l

@@ -85,7 +85,7 @@ extern const char *escaped_qstart, *escaped_qend;
 		reject = true;
 
 #define CHECK_YYREJECT(str) \
-	if ( all_lower( str ) ) \
+	if ( strncmp( str, "yyreject", 8 ) == 0 ) \
 		reject = true;
 
 #define YY_USER_INIT \