Local build improvement redo

[NQNStudios]

This is the last PR, with submodules instead of subtrees, and a few more things:

• add Catch2 dependency to the README
• partial build options
• correct debug flags on all platforms
• when scons -c is called, print a note about deleting .sconsign.dblite (this addresses No longer builds after scons -c #330, which I still can't reliably fix, but deleting the file manually works every time)