Resolve conflicts for functions introduced in Android API level 28. * m4/spawn_h.m4 (gl_HAVE_POSIX_SPAWN): Conditionally set REPLACE_POSIX_SPAWN. * lib/spawn.in.h: Adapt logic for defining the POSIX_SPAWN_* constants. (posix_spawn, posix_spawnp, posix_spawnattr_init, posix_spawnattr_destroy, posix_spawnattr_getsigdefault, posix_spawnattr_setsigdefault, posix_spawnattr_getsigmask, posix_spawnattr_setsigmask, posix_spawnattr_getflags, posix_spawnattr_setflags, posix_spawnattr_getpgroup, posix_spawnattr_setpgroup, posix_spawn_file_actions_init, posix_spawn_file_actions_destroy, posix_spawn_file_actions_addchdir, posix_spawn_file_actions_addfchdir): Disable _GL_CXXALIASWARN invocation on non-glibc systems.