[cmake-developers] [PATCH] Features: Only enable C11 support on GCC >= 4.9.

[Raphael Kubo da Costa]

As of version 3.3.1, CMake sets CMAKE_C11_STANDARD_COMPILE_OPTION and
CMAKE_C11_EXTENSION_COMPILE_OPTION for GCC >= 4.7, and checks for C11
features for GCC >= 4.6. It also means CMake itself will be built with
-std=gnu11 if GCC >= 4.7 is used.

However, GCC only has full C11 support with the 4.9 release according to
https://gcc.gnu.org/wiki/C11Status. Specifically, support for C11's
_Thread_local is only present on GCC >= 4.9. This combination makes
CMake fail to build with GCC 4.7 and 4.8 on FreeBSD, as runetype.h uses
_Thread_local and the fact that -std=gnu11 is passed makes it not be a
typedef or define for something else that would work.

Adjust the GCC feature detection code to only consider C11 support to
exist on GCC >= 4.9.

Bug: http://www.cmake.org/Bug/view.php?id=15741
---
 Modules/Compiler/GNU-C-FeatureTests.cmake | 7 +------
 Modules/Compiler/GNU-C.cmake              | 7 ++-----
 2 files changed, 3 insertions(+), 11 deletions(-)

diff --git a/Modules/Compiler/GNU-C-FeatureTests.cmake b/Modules/Compiler/GNU-C-FeatureTests.cmake
index b3fe33f..c7f5050 100644
--- a/Modules/Compiler/GNU-C-FeatureTests.cmake
+++ b/Modules/Compiler/GNU-C-FeatureTests.cmake
@@ -1,12 +1,7 @@
 
 set(_cmake_oldestSupported "(__GNUC__ * 100 + __GNUC_MINOR__) >= 404")
 
-# GNU 4.7 correctly sets __STDC_VERSION__ to 201112L, but GNU 4.6 sets it
-# to 201000L.  As the former is strictly greater than the latter, test only
-# for the latter.  If in the future CMake learns about a C feature which was
-# introduced with GNU 4.7, that should test for the correct version, similar
-# to the distinction between __cplusplus and __GXX_EXPERIMENTAL_CXX0X__ tests.
-set(GNU46_C11 "(__GNUC__ * 100 + __GNUC_MINOR__) >= 406 && defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201000L")
+set(GNU49_C11 "(__GNUC__ * 100 + __GNUC_MINOR__) >= 409 && defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L")
 set(_cmake_feature_test_c_static_assert "${GNU46_C11}")
 # Since 4.4 at least:
 set(GNU44_C99 "(__GNUC__ * 100 + __GNUC_MINOR__) >= 404 && defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L")
diff --git a/Modules/Compiler/GNU-C.cmake b/Modules/Compiler/GNU-C.cmake
index 031ab73..f1036db 100644
--- a/Modules/Compiler/GNU-C.cmake
+++ b/Modules/Compiler/GNU-C.cmake
@@ -14,12 +14,9 @@ if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.4)
   set(CMAKE_C99_EXTENSION_COMPILE_OPTION "-std=gnu99")
 endif()
 
-if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.7)
+if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.9)
   set(CMAKE_C11_STANDARD_COMPILE_OPTION "-std=c11")
   set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu11")
-elseif (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.6)
-  set(CMAKE_C11_STANDARD_COMPILE_OPTION "-std=c1x")
-  set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu1x")
 endif()
 
 if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 5.0)
@@ -34,7 +31,7 @@ macro(cmake_record_c_compile_features)
   endmacro()
 
   set(_result 0)
-  if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.6)
+  if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.9)
     _get_gcc_features(${CMAKE_C11_STANDARD_COMPILE_OPTION} CMAKE_C11_COMPILE_FEATURES)
   endif()
   if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.4)
-- 
2.5.2