closes bpo-13497: Fix broken nice configure test. (GH-12041)

Per POSIX, `nice(3)` requires `unistd.h` and `exit(3)` requires `stdlib.h`.

Fixing the test will prevent false positives with pedantic compilers like clang.

configure.ac

@@ -4727,6 +4727,8 @@ LIBS=$LIBS_no_readline
 AC_MSG_CHECKING(for broken nice())
 AC_CACHE_VAL(ac_cv_broken_nice, [
 AC_RUN_IFELSE([AC_LANG_SOURCE([[
+#include <stdlib.h>
+#include <unistd.h>
 int main()
 {
 	int val1 = nice(1);