Enable BUILD_PK3 by default

[charlesthobe]

Edit: now it doesn't have a useless flag that needs a PHD

Only touches the PK3 archive if it doesn't match the folder.

[charlesthobe]

and for Info-ZIP aka zip it's just -FS or --filesync
it does all of the above.

[charlesthobe]

Wait a sec, while zip doesn't touch the archive if there were no changes, 7zip still recreates the archive as slade.pk3.tmp, then deletes the original file and replaces it with the new file 🤦‍♂️

[charlesthobe]

I opted to create a cmake script instead of integrating the changes into the CMakeLists.txt so the command could be run in the build phase instead of the cmake configuration/cache generation, thus it won't be needed to regenerate cmake cache each time a file is changed.