build-sys: add check for gperf lookup function signature (#5055)

gperf-3.1 generates lookup functions that take a size_t length
parameter instead of unsigned int. Test for this at configure time.

Fixes: https://github.com/systemd/systemd/issues/5039

src/basic/cap-list.c

@@ -26,7 +26,7 @@
 #include "parse-util.h"
 #include "util.h"
 
-static const struct capability_name* lookup_capability(register const char *str, register unsigned int len);
+static const struct capability_name* lookup_capability(register const char *str, register GPERF_LEN_TYPE len);
 
 #include "cap-from-name.h"
 #include "cap-to-name.h"