I'm not a fan of Hungarian notation, but C++ kind of needs it with its
implicit `this`. And of all the common options for this, I find
suffixing members with an underscore the least offensive one.
We apparently didn't do that yet. SysEx should be super rare (outside of
octave switching on Arturia keyboards), but there's still no reason not
to do it.
We sadly cannot call `shared_library()` and `executable()` in these
subdirectories while still maintaining the same `build/` directory
structure, but this is still much cleaner. All of the other build
artifacts are now also gone from the root of `build/` so it's cleaner
overall.
Apparently this can actually make a difference in some cases, and the
C++ Core Guideliens recommend doing this on all default constructors,
destructors, and all functions that can not throw (and thus also don't
allocate).