updated build system

cmake/libraries/boost.cmake

@@ -95,9 +95,11 @@ if(PROJECT_ENABLE_BOOST)
       endif()
 
       if(PROJECT_ENABLE_LIBBITCOIN_SYSTEM)
-        set(BOOST_CXX_STANDARD 20)
+        set(BOOST_CXX_STANDARD 14)
+        set(BOOST_OPTIONS cxxflags=-Wno-enum-constexpr-conversion)
       else()
         set(BOOST_CXX_STANDARD ${CMAKE_CXX_STANDARD})
+        set(BOOST_OPTIONS define=BOOST_ASIO_HAS_STD_STRING_VIEW)
       endif()
 
       set(BOOST_BUILD_ARGS
@@ -106,14 +108,12 @@ if(PROJECT_ENABLE_BOOST)
         address-model=64
         architecture=${BOOST_ARCH}
         cxxstd-dialect=gnu
-        cxxflags=-std=gnu++${BOOST_CXX_STANDARD}
         cxxstd=${BOOST_CXX_STANDARD}
-        define=BOOST_ASIO_HAS_STD_STRING_VIEW
         define=BOOST_SYSTEM_NO_DEPRECATED
         link=${BOOST_LINK}
-        linkflags=-std=gnu++${BOOST_CXX_STANDARD}
         threading=multi
         variant=${BOOST_BUILD_TYPE_LOWER}
+        ${BOOST_OPTIONS}
       )
 
       ExternalProject_Add(boost_project