Builder|Linux: Label stable packages using the build number, too

To enable mixing unstable and stable packages, the build label is
included in both.
Builder: Determine new files based on modification time

Instead of naively looking at files that did not exist before, the
new output file detector now uses modification time as well to
determine if a file is new (and needs to be copied to the master).
Fixed|libdeng2: Compiler warning about truncating a double
libdeng2|VectorN<>: Equality operators for floating-point vector types
Merge branch 'master' into mapdata-c++