build:wafsamba: refactored the gcc-based dependency calculation

[github at samba.org]

New comment by obnoxxx on Samba Github repository

https://github.com/samba-team/samba/pull/31#issuecomment-156018099
Comment:
Generally, the changes look good. They work for me (tm).

On the cosmetic side, the patch that touches third_party/waf should not be labeled
'build:wafsamba' in the commit message, but rather 'third_party:waf: ...'.

I don't know much about gcc deps, so I can't say much about the actual backported changes.
Since they are alreay upstream in waf 1.5, I tend to accept them.
Is anyone willing to look deeper?
The second and third patch are certainly OK.