Add a MSVC2019_COMPATIBILITY flag to the conan recipe

[mscofield0]

The recipe fails to install on MSVC 2019 due to this flag being necessary.

[helmesjo]

Yeah I remember there being some issues regarding that which weren't trivial to circumvent automagically (requires some changes in the magnum library if I remember correctly (?)).

For now I'd just add the following before finding the package (I use the cmake_find_package generator, but the equivalent for any other generator would probably work):

set(CORRADE_MSVC2019_COMPATIBILITY ON)
find_package(magnum REQUIRED)

Edit
It's FindCorrade.cmake which expects that to be set, which is indirectly used by FindMagnum.cmake in my case.