FindThreads: add simple testcase

Tests/FindThreads/C-only/CMakeLists.txt

@@ -0,0 +1,10 @@
+cmake_minimum_required(VERSION 3.3 FATAL_ERROR)
+project(FindThreads_C-only C)
+
+set(CMAKE_THREAD_PREFER_PTHREAD On)
+find_package(Threads REQUIRED)
+
+if (NOT WIN32)
+  add_executable(thr ${CMAKE_CURRENT_SOURCE_DIR}/../../../Modules/CheckForPthreads.c)
+  target_link_libraries(thr Threads::Threads)
+endif ()

Tests/FindThreads/CMakeLists.txt

@@ -0,0 +1,9 @@
+add_test(NAME FindThreads.C-only COMMAND ${CMAKE_CTEST_COMMAND}
+  --build-and-test
+  "${CMake_SOURCE_DIR}/Tests/FindThreads/C-only"
+  "${CMake_BINARY_DIR}/Tests/FindThreads/C-only"
+  ${build_generator_args}
+  --build-project FindThreads_C-only
+  --build-options ${build_options}
+  --test-command ${CMAKE_CTEST_COMMAND} -V
+  )