libtins: Forcibly compile with C++14.

pkgs/development/libraries/libtins/0001-force-cpp-14.patch

···
+This change bypasses all the code that attempts to see which C++11 features are enabled in your specific C++11 compiler.  C++14 is required for gtest 1.13+.
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index 902233e676ee..49ac8a1010a4 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -103,17 +103,9 @@ ENDIF()
+ # C++11 support
+ OPTION(LIBTINS_ENABLE_CXX11 "Compile libtins with c++11 features" ON)
+ IF(LIBTINS_ENABLE_CXX11)
+-    # We only use declval and decltype on gcc/clang as VC fails to build that code,
+-    # at least on VC2013
+-    IF(HAS_CXX11_RVALUE_REFERENCES AND HAS_CXX11_FUNCTIONAL AND HAS_CXX11_CHRONO AND
+-       HAS_CXX11_NOEXCEPT AND ((HAS_CXX11_DECLVAL AND HAS_CXX11_DECLTYPE) OR MSVC))
+-        SET(TINS_HAVE_CXX11 ON)
+-        MESSAGE(STATUS "Enabling C++11 features")
+-        SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${CXX11_COMPILER_FLAGS}")
+-    ELSE()
+-        MESSAGE(WARNING "The compiler doesn't support the necessary C++11 features. "
+-                        "Disabling C++11 on this build")
+-    ENDIF()
++    SET(TINS_HAVE_CXX11 ON)
++    MESSAGE(STATUS "Using C++11 features")
++    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
+ ELSE(LIBTINS_ENABLE_CXX11)
+     MESSAGE(
+         WARNING

pkgs/development/libraries/libtins/default.nix

···
       url = "https://github.com/mfontanini/libtins/commit/812be7966d445ec56e88eab512f8fd2d57152427.patch";
       hash = "sha256-5RCFPe95r1CBrAocjTPR2SvUlgaGa1aBc8RazyxUj3M=";
     })
+    # Required for gtest 1.13+.
+    ./0001-force-cpp-14.patch
   ];
 
   postPatch = ''