Both LibGUI and ScummVM have a GUI::Widget class. This interferes with
normal operation of LibGUI, since the wrong GUI::Widget::~Widget() is
invoked when SerenitySDLWidget (from the SDL2 port) is destructed.
By adding `-fvisibility=hidden` to the compiler flags, we set the
symbol visibility to hidden by default and the correct destructor is
invoked again.
We may need entries with spaces in makeopts, installopts, and
configopts, and at that point we should also convert depends and
auth_opts to avoid confusion.
This unbreaks the ScummVM port build. Some `[[noreturn]]` keywords were
added to `<assert.h>` recently and this required an additional flag to
the ScummVM configure script to fix.
Also removed the now unnecessary `export LIBS`.
