posix_spawn is not available on Android devices.
While this commit is not necessary for cross-compiling, it is required
when building on-device since there posix_spawn is found somehow. This
produces following errors:
./spawn.h:514:17: error: use of undeclared identifier 'POSIX_SPAWN_RESETIDS'
[(((POSIX_SPAWN_RESETIDS | POSIX_SPAWN_SETPGROUP