Make warning CXXFLAGS configurable

This turns off -Werror by default except in the Debian package.
-Werror is inherently unportable and breaks things on OS X.  While it
can cause problems in Debian too, this is at least a little better.

It also turns off -pedantic in src/protobufs, because apparently
protoc output doesn’t compile with -pedantic on all platforms.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>