build fixes for non-standard opus/libsodium paths

On Mac OS X these are installed by homebrew in various subdirs under /usr/local/Cellar. The configure script can be told where these are explicitly, or via pkg-config. Unfortunately the arguments that pkg-config provides aren't being used correctly for libsodium. As for opus, the include path provided by pkg-config includes the 'opus' directory (e.g. '-I/usr/include/opus'), so you don't need to prefix the include directive with 'opus/'.

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