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When trying to use swig_add_library, configure step fails with CMake Error.

CMake Error: Error required internal CMake variable not set, cmake may not be built correctly.
Missing variable is:
CMAKE_CSharp_COMPILE_OBJECT

Minimal reproduction:

cmake_minimum_required(VERSION 3.12)

find_package(SWIG)
include(UseSWIG)

file(WRITE test.i "%module test")

swig_add_library(csharp
	TYPE MODULE
	LANGUAGE csharp
	SOURCES test.i)

Applying the following patch will fix the issue.

diff --git a/Modules/UseSWIG.cmake b/Modules/UseSWIG.cmake
index 8424a9ffc..02cd2bd4f 100644
--- a/Modules/UseSWIG.cmake
+++ b/Modules/UseSWIG.cmake
@@ -309,7 +309,7 @@ function(SWIG_GET_EXTRA_OUTPUT_FILES language outfiles generatedpath infile)
   foreach(it ${SWIG_${language}_EXTRA_FILE_EXTENSIONS})
     set(extra_file "${generatedpath}/${module_basename}${it}")
     if (extra_file MATCHES "\\.cs$")
-      set_source_files_properties(${extra_file} PROPERTIES LANGUAGE "CSharp")
+      set_source_files_properties(${extra_file} PROPERTIES LANGUAGE "")
     else()
       # Treat extra outputs as plain files regardless of language.
       set_source_files_properties(${extra_file} PROPERTIES LANGUAGE "")

I believe the issue will only appear on platforms that do not support building CSharp, i.e. everything but Windows.

Ref: !2137 (merged) and !2137 (0c2fb4c8, comment 420395)