csound: fix build (pthread not found) by adding stdenv.cc.libc.out/lib to -L. No idea why this is not default enough specifically when cmake is used

pkgs/applications/audio/csound/default.nix

···
 
   buildInputs = [ cmake libsndfile flex bison alsaLib libpulseaudio tcltk ];
 
+  NIX_LDFLAGS="-L${stdenv.cc.libc.out}/lib";
+
   meta = {
     description = "sound design, audio synthesis, and signal processing system, providing facilities for music composition and performance on all major operating systems and platforms";
     homepage = http://www.csounds.com/;