cmake: Prefer MSVC_RUNTIME_LIBRARY variable over manual CMAKE_CXX_FLAGS tweaking
This is more robust and less code on our side.
This also avoids the warning
omniORB4d.lib(GIOP_S.o) : error LNK2038: mismatch detected for 'RuntimeLibrary': value 'MTd_StaticDebug' doesn't match value 'MDd_DynamicDebug' in test_omniidl.obj
currently present in our MSVC builds.
The cmake policy 91 1 controlling this behaviour was introduced in cmake 3.15 and is therefore active by default.
We only support static MSVC library with static tango and dynamic MSVC library with dynamic tango, see 20bb9f47 (appveyor.yml: Fix MSVC runtime library selection, 2022-07-08), so it is okay to hardcode here.
Closes #1145 (closed)