Change Gconvert checkit() prototype for [perl #134371].

In the checkit() routine inside Configure, clang++ was taking the
if (strcmp(expect, got)) branch even though the 'expect' and 'got' strings
were identical.  A first step in debugging this was to realize that
the checkit() function never returned a value, so relabel it as void.
With clang version 7.0.1-8 (tags/RELEASE_701/final) (Debian),
this change seems to work around the strcmp issue.

Configure

@@ -10426,7 +10426,7 @@ char *myname = "sprintf";
 #endif
 #include <string.h>
 
-int checkit(char *expect, char *got)
+void checkit(char *expect, char *got)
 {
     if (strcmp(expect, got)) {
 		printf("%s oddity:  Expected %s, got %s\n",