Configure: fixed posix_spawn() detection with glic 2.30.

In particular, it was previously broken on Ubuntu 19.10 and Fedora 31.
See for details: https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=2ab5741

auto/unix

@@ -186,8 +186,8 @@ nxt_feature_libs=
 nxt_feature_test="#include <spawn.h>
                   #include <unistd.h>
 
-                  int main() {
+                  int main(int argc, char *argv[]) {
-                      (void) posix_spawn(NULL, NULL, NULL, NULL, NULL, NULL);
+                      (void) posix_spawn(NULL, \"\", NULL, NULL, argv, NULL);
                       return 0;
                   }"
 . auto/feature