debian: enable use of dpkg-buildflags if present

debian: enable use of dpkg-buildflags if present

On newer Debian-based distributions, dpkg-buildflags should be called
by the debian rules file to get the appropriate values for CFLAGS,
LDFLAGS, etc.  This will also allow Debian to build with security
hardened build flags, which is a release goal for Debian Wheezy.

If dpkg-buildflags is not present, then use some safe defaults (what
is currently being set by dpkg-buildpackage in Ubuntu 10.04 LTS, which
is a common enough ancient distribution that many people might still
care about).

Addresses-Debian-Bug: #654457

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>