Revamp how version tagging works

Rather than having it be a configurable option, simply build in final
release mode if Aegisub is being built from a signed tag, and get the
version information from that tag.

Drop the version number from the OS X app bundle when not building from
a tag as it done everywhere else. Drop the arch suffix since ppc hasn't
been tested in years and isn't worth testing as it's only going to get
less relevant as time goes on.

Remove some redundant definitions in the build system.