Handling hardening flags

[Guo Yixuan]

Hello,

I've cherry-picked most commits between 7.0.27+dfsg-4 and 7.0.27+dfsg-5 
in sid to master, in order to use the recommended way, dpkg-buildflags, 
to set various flags.

Because the default flags (-Werror=format-security) is too strict for 
boinc, it's disabled by
export DEB_BUILD_MAINT_OPTIONS = hardening=-format
(need some upstream effort to enable it)

(and this need /usr/share/dpkg/buildflags.mk to work, since exported 
variables don't take effect in $(shell dpkg-buildflags ...) )

Cheers,

Guo Yixuan