[llvm] [Offload] Skip plugins when none requested (PR #199901)

[Weronika Lewandowska via llvm-commits]

wlemkows wrote:

> > > Could you also add a description about the reationale? Like, is it just for printing the message or does `add_subdirectory(plugins-nextgen)` not work correctly without enabled plugins?
> > > Didn't you original change in #187006 also set some variables when plugins are skipped entirely?
> > 
> > 
> > If we merged #187006 The list of plugins may be empty because the host plugin isn't added by default on Windows. By default, the list will be empty. In that case, we shouldn't add the plugin-nextgen. Despite this, I don't think this change will break compatibility with the current implementation and can be merged first.
> 
> We made changes in #180401 so the host plugin would compile for Windows, and #189029 made it so it's always on the list of plugins, so the list can never be empty.

@adurang 
Thank you for the information. I didn't know this plugin was being ported because I encountered an error while building the host plugin on Windows. Do you have a solution for this?
`error C1189: #error:  “Missing preprocessor definitions for endianness detection.” [...]\runtimes\runtimes-bins\offload\plugins-nextgen\host\omptarget.rtl.host.vcxproj] [...]\runtimes\runtimes-build.vcxproj]`

https://github.com/llvm/llvm-project/pull/199901