build: make posix_spawn optional

OK, Android doesn't support it.

osdep/subprocess.c

@@ -17,6 +17,8 @@
 
 #include <pthread.h>
 
+#include "config.h"
+
 #include "common/common.h"
 #include "common/msg.h"
 #include "common/msg_control.h"
@@ -55,3 +57,13 @@ void mp_subprocess_detached(struct mp_log *log, char **args)
     if (pthread_create(&thread, NULL, run_subprocess, p))
         talloc_free(p);
 }
+
+#if !HAVE_SUBPROCESS
+int mp_subprocess(char **args, struct mp_cancel *cancel, void *ctx,
+                  subprocess_read_cb on_stdout, subprocess_read_cb on_stderr,
+                  char **error)
+{
+    *error = "unsupported";
+    return -1;
+}
+#endif

wscript

@@ -227,7 +227,6 @@ iconv support use --disable-iconv.",
         'desc': 'posix_spawnp() or MinGW',
         'func': check_true,
         'deps_any': ['posix-spawn', 'mingw'],
-        'req': True,
     }, {
         'name': 'glob',
         'desc': 'glob()',