message: gltfpack: Allow external customization of basisu executable path
  By default we assume it can be found in PATH but it's now possible to
  set BASISU_PATH to override this.
diff: @@ -2749,7 +2749,8 @@ bool encodeBasis(const std::string& data, std::string& result, bool normal_map,
  if (!writeFile(temp_input.c_str(), data))
  return false;
  - std::string cmd = "basisu";
  + const char* basisu_path = getenv("BASISU_PATH");
  + std::string cmd = basisu_path ? basisu_path : "basisu";
  char ql[16];
  sprintf(ql, "%d", (quality * 255 + 50) / 100);
  