Windows CI fails on cold dep build — MSVC rejects -Wno-enum-constexpr-conversion during openvdb compiler probe

[akenmorris]

openvdb dependency build with:

  cl : command line error D8021: invalid numeric argument
  '/Wno-enum-constexpr-conversion'
  [C:\bdeps\openvdb\build\CMakeFiles\CMakeTmp\cmTC_*.vcxproj]

The failure happens during CMakeTestCXXCompiler (the very first
compiler probe), so no openvdb source ever compiles.