pythongh-99086: Build: Fix -Wimplicit-int in PTHREAD_SCOPE_SYSTEM che…

…ck (python#99085)

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 <sam@gentoo.org>

Misc/NEWS.d/next/Build/2022-11-04-02-58-10.gh-issue-99086.DV_4Br.rst

@@ -0,0 +1 @@
+Fix ``-Wimplicit-int`` compiler warning in :program:`configure` check for ``PTHREAD_SCOPE_SYSTEM``.

configure

@@ -10947,7 +10947,7 @@ else
       void *foo(void *parm) {
         return NULL;
       }
-      main() {
+      int main() {
         pthread_attr_t attr;
         pthread_t id;
         if (pthread_attr_init(&attr)) return (-1);

configure.ac

@@ -3232,7 +3232,7 @@ 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);