configure.ac: Fix -Wimplicit-int in PTHREAD_SCOPE_SYSTEM check

Clang 16 makes -Wimplicit-int fatal by default.

Avoids errors like:
```
conftest.c:142:7: error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]
```

Signed-off-by: Sam James <[email protected]>

Author: thesamesam

configure.ac

@@ -3308,7 +3308,7 @@ if test "$posix_threads" = "yes"; then
       void *foo(void *parm) {
         return NULL;
       }
-      main() {
+      int main() {
         pthread_attr_t attr;
         pthread_t id;
         if (pthread_attr_init(&attr)) return (-1);