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/errno-list.c

@@ -23,7 +23,7 @@
 #include "macro.h"
 
 static const struct errno_name* lookup_errno(register const char *str,
-                                             register unsigned int len);
+                                             register GPERF_LEN_TYPE len);
 
 #include "errno-from-name.h"
 #include "errno-to-name.h"