Bug#821115: need emscripten for csdr

[Iain R. Learmonth]

block 821115 by 775147
block 821115 by 809997
kthxbye

Hi,

In order to build the JS version of csdr, as is used by openwebrx,
emscripten is required but this is currently broken in sid. Blocking the
ITP by the relevant emscripten bugs.

Thanks,
Iain.