Avoid using C11 to build CMake if _Thread_local support is broken

Support for C11's _Thread_local was introduced in GCC in the 4.9 series,
even though we make the C11 compiler flags available in CMake with GCC
>= 4.6.

FreeBSD's runetype.h uses _Thread_local, which causes CMake's own build
to fail when using GCC < 4.9 and -std=gnu11:

  /usr/include/runetype.h:92:22: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'const'
   extern _Thread_local const _RuneLocale *_ThreadRuneLocale;

Add a test for _Thread_local support and only build CMake itself with
C11 support if it works.

Bug: http://www.cmake.org/Bug/view.php?id=15741

CMakeLists.txt

@@ -38,7 +38,12 @@ endif()
 
 # Use most-recent available language dialects with GNU and Clang
 if(NOT DEFINED CMAKE_C_STANDARD AND NOT CMake_NO_C_STANDARD)
-  set(CMAKE_C_STANDARD 11)
+  include(${CMake_SOURCE_DIR}/Source/Checks/cm_c11_thread_local.cmake)
+  if(NOT CMake_C11_THREAD_LOCAL_BROKEN)
+    set(CMAKE_C_STANDARD 11)
+  else()
+    set(CMAKE_C_STANDARD 99)
+  endif()
 endif()
 if(NOT DEFINED CMAKE_CXX_STANDARD AND NOT CMake_NO_CXX_STANDARD)
   include(${CMake_SOURCE_DIR}/Source/Checks/cm_cxx14_cstdio.cmake)

Source/Checks/cm_c11_thread_local.c

@@ -0,0 +1,2 @@
+_Thread_local int i = 42;
+int main(void) { return 0; }

Source/Checks/cm_c11_thread_local.cmake

@@ -0,0 +1,33 @@
+set(CMake_C11_THREAD_LOCAL_BROKEN 0)
+if(CMAKE_CXX_COMPILER_ID MATCHES "GNU" AND CMAKE_C11_STANDARD_COMPILE_OPTION)
+  if(NOT DEFINED CMake_C11_THREAD_LOCAL_WORKS)
+    message(STATUS "Checking if compiler supports C11 _Thread_local")
+    try_compile(CMake_C11_THREAD_LOCAL_WORKS
+      ${CMAKE_CURRENT_BINARY_DIR}
+      ${CMAKE_CURRENT_LIST_DIR}/cm_c11_thread_local.c
+      CMAKE_FLAGS -DCMAKE_C_STANDARD=11
+      OUTPUT_VARIABLE OUTPUT
+      )
+    if(CMake_C11_THREAD_LOCAL_WORKS AND "${OUTPUT}" MATCHES "error: expected '=', ',', ';', 'asm' or '__attribute__' before 'int'")
+      set_property(CACHE CMake_C11_THREAD_LOCAL_WORKS PROPERTY VALUE 0)
+    endif()
+    if(CMake_C11_THREAD_LOCAL_WORKS)
+      message(STATUS "Checking if compiler supports C11 _Thread_local - yes")
+      file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeOutput.log
+        "Determining if compiler supports C11 _Thread_local passed with the following output:\n"
+        "${OUTPUT}\n"
+        "\n"
+        )
+    else()
+      message(STATUS "Checking if compiler supports C11 _Thread_local - no")
+      file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
+        "Determining if compiler supports C11 _Thread_local failed with the following output:\n"
+        "${OUTPUT}\n"
+        "\n"
+        )
+    endif()
+  endif()
+  if(NOT CMake_C11_THREAD_LOCAL_WORKS)
+    set(CMake_C11_THREAD_LOCAL_BROKEN 1)
+  endif()
+endif()