Treat windows gnullvm same as gnu for implib name

The implib filename is exactly the same as for gnu targets.
However, with current cargo versions, cargo will not expose the implib
on gnullvm targets and instead leaves the implib in the `deps`
subdirectory.

cmake/Corrosion.cmake

@@ -355,7 +355,7 @@ function(_corrosion_add_library_target)
         set(is_windows TRUE)
         if(Rust_CARGO_TARGET_ENV STREQUAL "msvc")
             set(is_windows_msvc TRUE)
-        elseif(Rust_CARGO_TARGET_ENV STREQUAL "gnu")
+        elseif(Rust_CARGO_TARGET_ENV STREQUAL "gnu" OR Rust_CARGO_TARGET_ENV STREQUAL "gnullvm")
             set(is_windows_gnu TRUE)
         endif()
     elseif(Rust_CARGO_TARGET_OS STREQUAL "darwin")