FindThreads: officially announce it works with only C++ enabled

Help/release/dev/Threads-CXX.rst

@@ -1,6 +1,6 @@
 Threads-CXX
 ------------
 
-* The :module:`CheckFunctionExists`, :module:`CheckLibraryExists`, and
+* The :module:`CheckFunctionExists`, :module:`CheckLibraryExists`,
-  :module:`CheckSymbolExists` modules learned to work in environments where
+  :module:`CheckSymbolExists`, and :module:`FindThreads` modules learned to
-  only CXX is enabled.
+  work in environments where only CXX is enabled.

Tests/FindThreads/CMakeLists.txt

@@ -1,9 +1,11 @@
-add_test(NAME FindThreads.C-only COMMAND ${CMAKE_CTEST_COMMAND}
+foreach (_lang IN ITEMS C CXX)
+  add_test(NAME FindThreads.${_lang}-only COMMAND ${CMAKE_CTEST_COMMAND}
     --build-and-test
-  "${CMake_SOURCE_DIR}/Tests/FindThreads/C-only"
+    "${CMake_SOURCE_DIR}/Tests/FindThreads/${_lang}-only"
-  "${CMake_BINARY_DIR}/Tests/FindThreads/C-only"
+    "${CMake_BINARY_DIR}/Tests/FindThreads/${_lang}-only"
     ${build_generator_args}
-  --build-project FindThreads_C-only
+    --build-project FindThreads_${_lang}-only
     --build-options ${build_options}
     --test-command ${CMAKE_CTEST_COMMAND} -V
     )
+endforeach ()

Tests/FindThreads/CXX-only/CMakeLists.txt

@@ -0,0 +1,13 @@
+cmake_minimum_required(VERSION 3.3 FATAL_ERROR)
+project(FindThreads_CXX-only CXX)
+
+set(CMAKE_THREAD_PREFER_PTHREAD On)
+find_package(Threads REQUIRED)
+
+configure_file(${CMAKE_CURRENT_SOURCE_DIR}/../../../Modules/CheckForPthreads.c
+               ${CMAKE_CURRENT_BINARY_DIR}/CheckForPthreads.cxx)
+
+if (NOT WIN32)
+  add_executable(thr ${CMAKE_CURRENT_BINARY_DIR}/CheckForPthreads.cxx)
+  target_link_libraries(thr Threads::Threads)
+endif ()