[CMake] Update required version of CMake to 3.9

.appveyor.yml

@@ -36,7 +36,7 @@ install:
   ############################################################################
   # Install a recent CMake
   ############################################################################
-  - set CMAKE_URL="https://cmake.org/files/v3.8/cmake-3.8.0-win64-x64.zip"
+  - set CMAKE_URL="https://cmake.org/files/v3.10/cmake-3.10.0-win64-x64.zip"
   - appveyor DownloadFile %CMAKE_URL% -FileName cmake.zip
   - 7z x cmake.zip -oC:\projects\deps > nul
   - move C:\projects\deps\cmake-* C:\projects\deps\cmake # Move to a version-agnostic directory

.travis.yml

@@ -229,7 +229,7 @@ install:
   ############################################################################
   - |
     if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
-      CMAKE_URL="https://cmake.org/files/v3.8/cmake-3.8.0-Linux-x86_64.tar.gz"
+      CMAKE_URL="https://cmake.org/files/v3.10/cmake-3.10.0-Linux-x86_64.tar.gz"
       mkdir cmake && travis_retry wget --no-check-certificate --quiet -O - ${CMAKE_URL} | tar --strip-components=1 -xz -C cmake
       export PATH=${DEPS_DIR}/cmake/bin:${PATH}
     else

CMakeLists.txt

@@ -2,7 +2,7 @@
 # Distributed under the Boost Software License, Version 1.0.
 # (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
 
-cmake_minimum_required(VERSION 3.8)
+cmake_minimum_required(VERSION 3.9)
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 
 ##############################################################################
@@ -50,15 +50,10 @@ include(CheckCxxCompilerSupport)
 include(CheckCXXCompilerFlag)
 add_library(hana INTERFACE)
 target_include_directories(hana INTERFACE "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>")
-# With Clang on Windows, the -std=c++14 flag is incorrectly set and the compiler
-# complains about the unknown option. TODO: Remove this workaround once the
-# underlying bug is fixed in CMake: https://gitlab.kitware.com/cmake/cmake/issues/17015
-if (NOT (MSVC AND ${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang"))
-    if (BOOST_HANA_ENABLE_CPP17)
-        target_compile_features(hana INTERFACE cxx_std_17)
-    else()
-        target_compile_features(hana INTERFACE cxx_std_14)
-    endif()
+if (BOOST_HANA_ENABLE_CPP17)
+    target_compile_features(hana INTERFACE cxx_std_17)
+else()
+    target_compile_features(hana INTERFACE cxx_std_14)
 endif()
 
 # Export the `hana` library into a HanaConfig.cmake file