examples: CMake: don't build core_loading_thread if no pthreads

The example requires pthreads and now C11 <stdatomic.h>, thus skip it
if either isn't available.

This also fixes the current Travis CI build failure for the
Windows configuration.

examples/CMakeLists.txt

@@ -28,6 +28,24 @@ include(CheckIncludeFiles)
 set(OUTPUT_EXT)
 list(REMOVE_ITEM example_sources ${CMAKE_CURRENT_SOURCE_DIR}/others/rlgl_standalone.c)
 
+include(CheckIncludeFile)
+CHECK_INCLUDE_FILE("stdatomic.h" HAVE_STDATOMIC_H)
+set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
+find_package(Threads)
+if (CMAKE_USE_PTHREADS_INIT AND HAVE_STDATOMIC_H)
+  add_if_flag_compiles("-std=c11" CMAKE_C_FLAGS)
+  if(THREADS_HAVE_PTHREAD_ARG)
+    add_if_flag_compiles("-pthread" CMAKE_C_FLAGS)
+  endif()
+  if(CMAKE_THREAD_LIBS_INIT)
+    link_libraries("${CMAKE_THREAD_LIBS_INIT}")
+  endif()
+else()
+  # Items requiring pthreads
+  list(REMOVE_ITEM example_sources ${CMAKE_CURRENT_SOURCE_DIR}/core/core_loading_thread.c)
+endif()
+
+
 if(${PLATFORM} MATCHES "Android")
   list(REMOVE_ITEM example_sources ${CMAKE_CURRENT_SOURCE_DIR}/others/rlgl_standalone.c)
   list(REMOVE_ITEM example_sources ${CMAKE_CURRENT_SOURCE_DIR}/others/standard_lighting.c)