packages gcc libvpx openssl portaudio: use escaped substitution style

prompted by sample package recipe on @TimothyGu's wiki
Merge commit '7d7dd227db6612ac3aa555fec236907964404878' into my

* commit '7d7dd227db6612ac3aa555fec236907964404878':
  packages gcc libvpx openssl portaudio: use escaped substitution style