bugfix: we require ALSA
bugfix: remove leftovers from scons platform checks

...this becomes relevant when you abort an build and
then try to start the package building from scratch.
Build/bugfix: always link agrainst lib rt explicitly

we need the function clock_gettime, which sometimes
is available from the standard library; but for sake
of sane build dependencies we should declare an explicit
dependency to librt by linking against it
32/64 platform fixes prompted by failing tests

some of these fixes are preliminary workarounds;
our approach for dealing with frame numbers isn't consistent;
problems happen at places where we used 'long' while we shouldn't