Add a build option to link with the static CRT

[mikejsavage]

I'm not 100% sure if this is the right approach but atm you can't link Luau into an app that uses /MT(d)

[zeux]

Do you need this for CMake versions older than 3.15? The handling of this in earlier versions seems a little messy. For 3.15 to work I think you need to opt into CMP0091 like this:

if(POLICY CMP0091)
    cmake_policy(SET CMP0091 NEW) # Enables use of MSVC_RUNTIME_LIBRARY
endif()

[zeux]

Perfect, thanks! I double checked locally and this appears to work well on Luau project at least.

[mikejsavage]

Thanks for the quick reviews, to be honest I don't really know cmake so my first commit was copy pasted from curl

I've changed it to just use CMAKE_MSVC_RUNTIME_LIBRARY. It's a little awkward because you have to set the policy before you call project, and MSVC doesn't get set until you call project, so now it's a valid option for all platforms.

If that's not ok then another approach would be to use if(POLICY) to unconditionally set the policy, something like

if(POLICY CMP0091)
    cmake_policy(SET CMP0091 NEW)
endif()
...
project(Luau ...)
...
if(MSVC)
    option(LUAU_STATIC_CRT "Link with the static CRT (/MT)" OFF)
    if(LUAU_STATIC_CRT)
        set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
    endif()
endif()

Btw if this is too much back and forth for a ~5 line change then feel free to just close this/resolve it without me

[zeux]

I think it's fine to set up the project after options, if we need to move it back we can do this separately