CMake: let CMake handle the C++ standard setting if target_compile_fe…

…atures() is used

This raises the required CMake version for this branch to get a better error
message. target_compile_features() was introduced in CMake 3.1.

src/CMakeLists.txt

@@ -249,10 +249,11 @@ if (use_libclang)
     find_package(LLVM REQUIRED CONFIG)
     find_package(Clang REQUIRED CONFIG)
     if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+        cmake_minimum_required(VERSION 3.1)
         target_compile_features(_doxygen PRIVATE cxx_alignof)
         target_compile_features(doxygen PRIVATE cxx_alignof)
-        target_compile_options(_doxygen PRIVATE -stdlib=libc++ -std=c++11)
-        target_compile_options(doxygen PRIVATE -stdlib=libc++ -std=c++11)
+        target_compile_options(_doxygen PRIVATE -stdlib=libc++)
+        target_compile_options(doxygen PRIVATE -stdlib=libc++)
     elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
         target_compile_options(_doxygen PRIVATE -std=c++11)
         target_compile_options(doxygen PRIVATE -std=c++11)