CMakeLists.txt: Enable CMAKE_MSVC_RUNTIME_LIBRARY support

[t-b]

The documentation for CMAKE_MSVC_RUNTIME_LIBRARY states 1:

This variable has effect only when policy CMP0091 is set to NEW prior to
the first project() or enable_language() command that enables a language
using a compiler targeting the MSVC ABI.

so the current usage of CMAKE_MSVC_RUNTIME_LIBRARY for vcpkg does not work at all.

Let's fix that by setting policy 91 to new if present.

[lalitb]

Sorry, removing the approval for the CI failure. Probably we should set the policy only if vcpkg is used (i.e, VCPKG_TOOLCHAIN is set)? cc @ThomsonTan

[t-b]

Sorry, removing the approval for the CI failure. Probably we should set the policy only if vcpkg is used (i.e, VCPKG_TOOLCHAIN is set)? cc @ThomsonTan

My usecase is not related to vcpkg, I'm just compiling with MSVC.

[marcalff]

Thanks for the PR.

This change is probably not sufficient, as it triggers build failures in CI.

Please investigate and fix errors such as:

benchmark.lib(benchmark.cc.obj) : error LNK2038: mismatch detected for 'RuntimeLibrary': value 'MDd_DynamicDebug' doesn't match value 'MTd_StaticDebug' in attributes_hashmap_benchmark.obj [D:\a\opentelemetry-cpp\opentelemetry-cpp\build\sdk\test\metrics\attributes_hashmap_benchmark.vcxproj]

[t-b]

Thanks for the PR.

This change is probably not sufficient, as it triggers build failures in CI.

Please investigate and fix errors such as:

benchmark.lib(benchmark.cc.obj) : error LNK2038: mismatch detected for 'RuntimeLibrary': value 'MDd_DynamicDebug' doesn't match value 'MTd_StaticDebug' in attributes_hashmap_benchmark.obj [D:\a\opentelemetry-cpp\opentelemetry-cpp\build\sdk\test\metrics\attributes_hashmap_benchmark.vcxproj]

Thanks for the review, will do.