Update buildtype from debug and optimization

[bruchar1]

By adding a flag to indicate whether an option was explicitly set or not, it is now possible to update the value of buildtype from the values of debug and optimization, without modifying options that are explicitly set.

Fixes #11645
Fixes #10487
Fixes #5814 (maybe this one was already fixed, but at least a test proves it works)

[jpakkane]

FWICT the description means that if you set, for example, optimization to 2 and debug to true, it would update buildtype to debugoptimized. If that is the case, then I don't really like this behaviour. The linked documentation says that buildtype sets opt and debug and "vice versa". It really should not say that (or do it, for that matter).

Buildtype is conceptually at a higher level than optimization and debug options. It means, roughly, "the current build setup will be used for purpose X, configure yourself accordingly". Resetting the latter to "expected defaults" when changing build type is reasonable and expected, whereas the opposite is not.

If we were starting from scratch, then having a build dir set up for debugoptimized and changing optimization level from 2 to, say, 3 should be permitted and not change the build type. This is in fact needed, since minsize might require the use of optimization level s, z, or even 2. The former means "configure your source code to use algorithms etc that minimized code size" whereas the latter is merely tells the compiler "optimized based on target property X".

The way the documentation specifies things is not good and we should change it to this more sensible approach if we can get away with it without breaking backwards compatibility too much.

[bruchar1]

The subtilty is that buildtype will be set from debug and optimization only if it is not explicitly set. It doesn't prevent setting a different optimization when buildtype is explicitly set.