blackmagic: fix darwin build

pkgs/development/embedded/blackmagic/default.nix

···
     homepage = "https://github.com/blacksphere/blackmagic";
     license = licenses.gpl3Plus;
     maintainers = with maintainers; [ pjones emily sorki ];
-    # fails on darwin with
-    # arm-none-eabi-gcc: error: unrecognized command line option '-iframework'
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }

pkgs/development/embedded/blackmagic/helper.sh

···
 out=${out:-/tmp}
 
 ################################################################################
-export CFLAGS=$NIX_CFLAGS_COMPILE
 export MAKEFLAGS="\
   ${enableParallelBuilding:+-j${NIX_BUILD_CORES}}"