scons: always set __STDC_*_MACROS for C++ sources

Analogous to previous commit - just set the lot once throughout.

Cc: Jose Fonseca <jfonseca@vmware.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
2 files changed