cmake: set CMP0074 to NEW (for OPENSSL_ROOT in appveyor)

cmake: set CMP0074 to NEW (for OPENSSL_ROOT in appveyor)

We have $env:OPENSSL_ROOT (env) equals to -DOPENSSL_ROOT (cmake
variable) anyway.

cmake complains:
  Policy CMP0074 is not set: find_package uses <PackageName>_ROOT variables.
  Run "cmake --help-policy CMP0074" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  Environment variable OpenSSL_ROOT is set to:
    C:/OpenSSL-Win64/bin

  For compatibility, CMake is ignoring the variable.
This warning is for project developers.  Use -Wno-dev to suppress it.

(cherry picked from commit 65904773f2bf965e2050d4d7c91e30d4f123a787)