gh-99086: Build: Fix -Wimplicit-int in PTHREAD_SCOPE_SYSTEM check (#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 <[email protected]>

Author: thesamesam

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

@@ -0,0 +1 @@
+Fix ``-Wimplicit-int`` warning in configure.ac when testing for PTHREAD_SCOPE_SYSTEM.

configure.ac

@@ -3367,7 +3367,7 @@ 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);