build-sys: add namespace.h to distribution

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
build-sys: test for secure_getenv() too.

In current glibc versions, internal __secure_getenv
no longer exists and was replaced by secure_getenv()

Signed-off-by: Cristian Rodríguez <crrodriguez@opensuse.org>