-Wall -Werror makes qtprotobuf depend on exact compiler and version

[pontaoski]

-Wall -Werror isn't an option that should be set by anyone other than developers on their own machines, as warnings aren't stable across compilers and versions.

What may be a valid program with one compiler version under -Wall -Werror fails to compile with another compiler and version. This goes both directions: older compilers than what the author developed for may take issue, and newer compilers than what the author developed for may also take issue, resulting in much pain whenever the author and user aren't using the same compiler version.