message: [vcpkg] Add a definition for OVERLAY_TRIPLET_ENV
  This definition is needed in C++14/gcc 6 to compile.
diff: @@ -912,6 +912,7 @@ namespace vcpkg
  constexpr StringLiteral VcpkgCmdArguments::OVERLAY_PORTS_ENV;
  constexpr StringLiteral VcpkgCmdArguments::OVERLAY_PORTS_ARG;
  constexpr StringLiteral VcpkgCmdArguments::OVERLAY_TRIPLETS_ARG;
  + constexpr StringLiteral VcpkgCmdArguments::OVERLAY_TRIPLETS_ENV;
  constexpr StringLiteral VcpkgCmdArguments::BINARY_SOURCES_ARG;
  