lex: %option not supported by solaris lex

it turns out that we don't need to tell lex we don't plan to use unput;
we can just.... not use unput. however, if we're flex, use the command
line option if it's available, to avoid warnings

lib/sl/Makefile.am

@@ -9,6 +9,7 @@ endif
 AM_CPPFLAGS += $(ROKEN_RENAME)
 
 YFLAGS = -d
+LFLAGS = @FLEXNOUNPUTARGS@
 
 include_HEADERS = sl.h
 

lib/sl/slc-lex.l

@@ -53,8 +53,6 @@ static char * handle_string(void);
 
 %}
 
-%option nounput
-
 %%
 [A-Za-z][-A-Za-z0-9_]*	{
 			  yylval.string = strdup ((const char *)yytext);