Don't use exit in PCAP_IS_SUITABLE.

John Spencer reported that musl libc doesn't automatically include
<stdlib.h>, as Glibc does, so the configure check was wrongly failing.

conftest.c: In function 'main':
conftest.c:35:5: error: implicit declaration of function 'exit'

configure

@@ -6313,12 +6313,12 @@ int main() {
   int major, minor1, minor2;
   sscanf(pcap_version,"%d.%d.%d", &major, &minor1, &minor2);
   if (major > 0)
-    exit(0);
+    return 0;
   if (minor1 < 9)
-    exit(1);
+    return 1;
   if (minor2 < 4)
-    exit(1);
-  exit(0);
+    return 1;
+  return 0;
 }
 
 _ACEOF