Don't downgrade the C++ version

The cmake configuration adds a compiler flag to set the C++ version to
C++11. This happens even if the default C++ version for the GCC
compiler version used is higer than C++11. In this case the flag
downgrades the version.

Since system libraries and headers can use features compatible with
the default C++ version of the default compiler, downgrading the C++
version might make some system libraries incompatible with this
setting.

The recent update of googletest in Fedora requires C++ 14.
The gcc version in Fedora Rawhide is currently 13.

C++ 17 is the default since GCC 11.
C++ 14 is the default since GCC 6.1.

This PR drops the addition of the -std=c++11 flag if the GCC version
is 6.1 or greater. With this change the code can be compiled on Fedora
Rawhide without triggering errors:

/usr/include/gtest/internal/gtest-port.h:270:2: error: #error C++
versions less than C++14 are not supported.

The implementation and the preceding commit message text are by Mattias
Ellert, @ellert, copied from cern-fts/davix#103.

cmake/modules/CMakeCXX11Support.cmake

@@ -3,13 +3,16 @@ include(CheckCXXSourceCompiles REQUIRED)
 
 if(CMAKE_COMPILER_IS_GNUCXX)
    execute_process(COMMAND ${CMAKE_C_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION)
-   if (GCC_VERSION VERSION_GREATER 4.7 OR GCC_VERSION VERSION_EQUAL 4.7)
-        SET(HAVE_CXX011_FULL_SUPPORT TRUE)
-        SET(HAVE_CXX011_PARTIAL_SUPPORT TRUE)
-                SET(CXX11_FLAG_ENABLE "-std=c++11")
+   if(GCC_VERSION VERSION_GREATER 6.1 OR GCC_VERSION VERSION_EQUAL 6.1)
+		SET(HAVE_CXX011_FULL_SUPPORT TRUE)
+		SET(HAVE_CXX011_PARTIAL_SUPPORT TRUE)
+   elseif(GCC_VERSION VERSION_GREATER 4.7 OR GCC_VERSION VERSION_EQUAL 4.7)
+		SET(HAVE_CXX011_FULL_SUPPORT TRUE)
+		SET(HAVE_CXX011_PARTIAL_SUPPORT TRUE)
+		SET(CXX11_FLAG_ENABLE "-std=c++11")
    elseif(GCC_VERSION VERSION_GREATER 4.3 OR GCC_VERSION VERSION_EQUAL 4.3)
         message(STATUS "C++11 partial support")
-        SET(HAVE_CXX011_PARTIAL_SUPPORT TRUE)
+		SET(HAVE_CXX011_PARTIAL_SUPPORT TRUE)
                 SET(CXX11_FLAG_ENABLE "-std=c++0x")
    else ()
         message(STATUS "C++11 no support ")
@@ -22,7 +25,6 @@ else(CMAKE_COMPILER_IS_GNUCXX)
    SET(CXX11_FLAG_ENABLE "-std=c++0x")
 endif(CMAKE_COMPILER_IS_GNUCXX)
 
-
 ## Check TR1
 CHECK_CXX_SOURCE_COMPILES("
 #include <tr1/functional>
@@ -34,4 +36,3 @@ message(STATUS "TR1 support detected")
 else(HAVE_TR1_SUPPORT)
 message(STATUS "no TR1 support")
 endif(HAVE_TR1_SUPPORT)
-