nptl: do not define _POSIX_SPAWN since its not implemented
Building things like vlc checks if _POSIX_SPAWN is defined. Since
posix_spawn is not implemented we dont define it.
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>