VCPKG sets CMAKE_CXX_FLAGS and CMAKE_C_FLAGS automatically; Apparently our code to set those flags has never worked and rather than ignoring most of it, it changed to putting it somewhere it doesn't belong.
This will build the very latest Opus available (May 30 2020). The absolute
minimum version of Opus is commit 927de8453c502586c03e25c169ec08f2a93ebc02,
released on April 21, 2020.
This incorporates a fix for xiph/opus#154