cmake: support MSVC_RUNTIME_LIBRARY property

[compnerd]

When using a new enough CMake (3.15+) prefer to use the
MSVC_RUNTIME_LIBRARY property on targets to select the runtime
library variant. This property is automatically set to the value
specified by CMAKE_MSVC_RUNTIME_LIBRARY. This proper requires that
the CMake Policy 91 is set to new (see CMP0091).

[compnerd]

CC: @haberman

[compnerd]

Anything else left before we could merge this @haberman?

[jtattermusch]

This PR seems to be breaking gRPC build in a test job that tried to continuously test against protobuf's head.

https://source.cloud.google.com/results/invocations/7ea8aefa-6622-47c6-9c2f-092b0cc87f7c/targets/github%2Fgrpc%2Ftoplevel_run_tests_invocations%2Frun_tests_c_linux_dbg_native_buildonly/tests

internal b/198323179

-- 3.17.3.0
CMake Error at third_party/protobuf/cmake/CMakeLists.txt:187 (if):
  if given arguments:

    "CMAKE_VERSION" "VERSION_GREATER_EQUAL" "3.15"

  Unknown arguments specified

Looks like this change might be broken? Should we revert?

[compnerd]

I don't think that this change is broken. The problem is likely the CMake version that is being used to build.

[snnn]

If I understand correctly, when protobuf_BUILD_SHARED_LIBS is OFF and the latest cmake is in-use, this change will force users using static CRT? Previously at least they have a protobuf_MSVC_STATIC_RUNTIME for us to choose. The line

    set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreaded$<$<CONFIG:Debug>:Debug>)

shouldn't exist at all.