This is basically the same issue as in tests/various/plugin.sh,
which uses yosys-config to compile a plugin. `yosys-config --cxxflags`
points to `$PREFIX/share/` (/usr/local/share by default), which might
not exist yet or might be out of date. Building directly from the
headers in ./share/ avoids this.
o Not all derived methods were marked 'override', but it is a great
feature of C++11 that we should make use of.
o While at it: touched header files got a -*- c++ -*- for emacs to
provide support for that language.
o use YS_OVERRIDE for all override keywords (though we should probably
use the plain keyword going forward now that C++11 is established)