gcc emits `Wsubobject-linkage`, because the the publicly visible
`TablePrinter` contains `ColumnInfo`, which is part of an anonymous
namespace

Classes will automatically inherit the virtual-ness of their base
class destructors. If the base class already has a virtual
destructor and the derived class needs default destructor semantics
then the derived class can omit defining the destructor in favor of
the compiler automatically defining it.

This has an additional benefit of reenabling move semantics. The
presence of a user-specified destructor automatically disables move
operations.

This is the recommended way of adding new Opts in our documentation
for using custom main, but we did not compile the code to see if it
works. We now compile the example as part of the BUILD_EXAMPLES
option.

Fixes #2787

* Alter meson for Android cross builds.

* Handle liblog linking on Termux as well as the Android NDK.

This removes about 200 pointless copies from printing the help
message (the original motivation for the change), and also nicely
improves performance of the various reporters that depend on
TextFlow.

The code is now even worse mess than before, due to the ad-hoc
implementation of Result-ish type based on virtual functions in
Clara, but it has dropped the allocations for empty binary down
to 151.