libtins: Forcibly compile with C++14.

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

      url = "https://github.com/mfontanini/libtins/commit/812be7966d445ec56e88eab512f8fd2d57152427.patch";

      hash = "sha256-5RCFPe95r1CBrAocjTPR2SvUlgaGa1aBc8RazyxUj3M=";

    })

    # Required for gtest 1.13+.

    ./0001-force-cpp-14.patch

  ];

  postPatch = ''