I tried the VB6 runtime install:
The gets the Mapper to load a bit further. It then fails trying to load components.
I tried the component registration for OP2Mapper (https://forum.outpost2.net/index.php?topic=4066.0):
regsvr32 cpopmenu6.ocx
regsvr32 vbaldtab6.ocx
regsvr32 vbaltbar6.ocx
regsvr32 vbaltreeview6.ocx
Unfortunately I seem to be missing one of the controls:
regsvr32: Failed to load DLL 'vbaldtab6.ocx'
I'm not sure what to do about that. I assume that control somehow got left out of the packaging.
Mingw is already setup as a cross compiler, and comes packaged with Windows header files. You pretty much just call it, though you will need to translate MSVC project specific settings to Mingw command line options.
Edit: I use the following alias for the below commands:
alias mingw='i686-w64-mingw32-g++'
For reference, I was able to use this with Brett's Yukon Trail mission:
mingw -I OP2MissionSDK/ *.cpp
The -I is to set an additional include directory for the SDK.
As Mingw is not able to link due to the name mangling differences, the above would produce a ton of linker errors after the compile step. If you want to avoid the spurious errors and just do syntax checking, you could use:
mingw -fsyntax-only -I OP2MissionSDK/ *.cpp
I came across this page for Clang Cross Compilation (https://clang.llvm.org/docs/CrossCompilation.html).
To get setup with a Windows Virtual Machine, Microsoft can help with that:
Get a Windows 10 development environment (https://developer.microsoft.com/en-us/windows/downloads/virtual-machines)
It's a free download (time limited). A bit large though, about 16GB download, and 20GB unpacked.
If you want to keep using it past the time limit, I believe you'd be able to buy a licence and register it with the VM. It should also be possible to redownload/reinstall at the end of the trial period.