[CMake] Simplify cmake file regarding filesystem library

CMakeLists.txt

@@ -4,46 +4,19 @@
 # See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt
 
 cmake_minimum_required( VERSION 3.1 )
-if(${CMAKE_VERSION} VERSION_GREATER "3.8.0")
-    # Honor CXX_STANDARD_REQUIRED in check_cxx_source_compiles
-    # (see https://cmake.org/cmake/help/v3.8/policy/CMP0067.html)
-    cmake_policy( SET CMP0067 NEW )
-endif()
 
 project( boostdep LANGUAGES CXX )
 
-# Compile with c++17 if available (so we can use std::filesystem),
-# but boostdep is c++98 compatible, so no CMAKE_CXX_STANDARD_REQUIRED is necessary
-set( CMAKE_CXX_STANDARD 17 )
+option( BOOSTDEP_USE_STD_FS "Use std::filesystem instead of boost::filesystem" OFF )
 
 add_executable( boostdep src/boostdep.cpp )
 
-####### options #######
-
-# Decide if boostdep should use Boost.Filesystem or std::filesystem
-
-# We default to using the standard library when available
-# and Boost.Filesystem otherwise
-# NOTE: Prior to 3.8.0 check_cxx_source_compiles will not honor the c++17 setting,
-#        so with older cmake versions this will never detect std::filesystem correctly
-include( CheckCXXSourceCompiles )
-check_cxx_source_compiles(
-"
-#include <filesystem>
-int main() {
-    std::filesystem::is_directory( std::filesystem::current_path() );
-}
-"
-has_std_filesystem )
-
-option( BOOSTDEP_USE_STD_FS "Use std::filesystem instead of boost::filesystem" ${has_std_filesystem} )
-
-#~~~~~~ options ~~~~~~
-
 if( BOOSTDEP_USE_STD_FS )
 
     message(STATUS "Using std::filesystem as filesystem library")
+
     target_compile_definitions( boostdep PUBLIC BOOSTDEP_USE_STD_FS )
+    target_compile_features( boostdep PUBLIC cxx_std_17 )
 
     # NOTE: Some compilers (e.g. g++-8) will require additional linker flags
     #        in order to use std::filesystem (e.g. -lstdc++fs).
@@ -58,5 +31,4 @@ else()
 
 endif()
 
-
 install( TARGETS boostdep RUNTIME DESTINATION bin )