configure: Detect type of length parameter for gperf generated function

Since 3.1 gperf uses size_t for the length parameter instead of an
unsigned int.

src/stroke/stroke_keywords.h

@@ -73,6 +73,6 @@ typedef enum {
 
 typedef struct stroke_token stroke_token_t;
 
-extern const stroke_token_t* in_word_set(register const char *str, register unsigned int len);
+extern const stroke_token_t* in_word_set(register const char *str, register GPERF_LEN_TYPE len);
 
 #endif /* _STROKE_KEYWORDS_H_ */