dpkg-buildpackage: Set a set of compiler flags for a build

dpkg-buildpackage: Set a set of compiler flags for a build

* scripts/dpkg-buildpackage.pl: Set a set of environment variables
for setting compiler and linker options, unless already set in the
environment. See https://wiki.ubuntu.com/DistCompilerFlags for
background information.
* man/dpkg-buildpackage.pl: Document the new behaviour.

Based on a patch by Matthias Klose <doko@cs.tu-berlin.de>