Add CFLAGS_FOR_TARGET to Ada OS Constant Extraction Process

[Arnaud Charlet]

> When building Ada for RTEMS, you need to pass in the CFLAGS_FOR_TARGET
> to see OS specific .h files. This was already done for the RTS but needed to
> be added to the process which extracts value settings from the .h files.
> 
> This patch is against 4.9.  OK to apply to 4.8, 4.9, and head assuming it
> applies cleanly?

This patch won't apply cleanly for head I think, where I suspect at least
part of your patch is OBE.

It would be better to first have a patch for trunk (if still relevant)
and then backport it to the other branches, since changes on trunk have
already been done in a different way to address at least partly this issue
(e.g. use of GNATLIBCFLAGS_FOR_C in OSCONS_CPP, different computation of
OSCONS_CC).

Arno