Work on a complete iOS boost compile script that rolls up all the necessary architectures

boost.cmake

include( ExternalProject )

set( NCPU 8 )
set( MIN_IOS_VERSION 8.0 )
set( BOOST_VERSION 1.63.0 )
set( BOOST_SHA_CHECKSUM 9f1dd4fa364a3e3156a77dc17aa562ef06404ff6 )
set( BOOST_COMPILE_LIBRARIES filesystem system date_time )
set( COMPILE_ARCHITECTURES armv6 armv7 armv7s arm64 )
set( ENABLE_BITCODE true )

string( REPLACE "." "_" BOOST_FILENAME "boost_${BOOST_VERSION}" )

set( boost_URL "http://sourceforge.net/projects/boost/files/boost/${BOOST_VERSION}/${BOOST_FILENAME}.tar.bz2" )
set( boost_INSTALL ${CMAKE_CURRENT_BINARY_DIR}/third_party/boost )
set( boost_INCLUDE_DIR ${boost_INSTALL}/include )
set( boost_LIB_DIR ${boost_INSTALL}/lib )

execute_process( COMMAND xcrun --sdk iphoneos --show-sdk-platform-path
	COMMAND tr -d '\r\n'
    OUTPUT_VARIABLE XCODE_ROOT )
    
execute_process( COMMAND xcrun --sdk iphoneos --find clang++
	COMMAND tr -d '\r\n'
	OUTPUT_VARIABLE CLANG_PATH )

execute_process( COMMAND xcrun --sdk iphoneos --show-sdk-path
    COMMAND tr -d '\r\n'
    OUTPUT_VARIABLE XCODE_SYSROOT )
    
execute_process( COMMAND xcodebuild -showsdks
	COMMAND grep iphoneos 
	COMMAND egrep "[[:digit:]]+\.[[:digit:]]+" -o
	COMMAND tail -1
	COMMAND tr -d '\r\n'
	OUTPUT_VARIABLE IOS_SDK_VERSION )

message( STATUS "IOS SDK Version ${IOS_SDK_VERSION}" )

string( REPLACE ";" "," BOOST_WITH_LIBRARIES "${BOOST_COMPILE_LIBRARIES}" )
message( STATUS "Compile Boost libraries ${BOOST_WITH_LIBRARIES}" )

set( COMPILEFLAGS )
foreach( COMPILE_ARCHITECTURE ${COMPILE_ARCHITECTURES} )
    set( COMPILEFLAG "<compileflags>\"-arch ${COMPILE_ARCHITECTURE}\"" )
	list( APPEND COMPILEFLAGS ${COMPILEFLAG} )
endforeach( COMPILE_ARCHITECTURE )

if( ENABLE_BITCODE )
list( APPEND COMPILEFLAGS "<compileflags>-fembed-bitcode" )
endif( ENABLE_BITCODE )
string( REPLACE ";" "\n" COMPILEFLAGS_STRING "${COMPILEFLAGS}" )

set( USER_CONFIG_JAM
"using darwin : ${IOS_SDK_VERSION}~iphone :
${CLANG_PATH} :
<striper>
<root>${XCODE_ROOT}/Developer
<compileflags>-mios-version-min=${MIN_IOS_VERSION}
<compileflags>-std=c++11
<compileflags>-stdlib=libc++
<compileflags>-fvisibility-inlines-hidden
<compileflags>--sysroot=${XCODE_SYSROOT}
${COMPILEFLAGS_STRING}
\;
" )
file( WRITE ${CMAKE_BINARY_DIR}/user-config.jam ${USER_CONFIG_JAM} )

ExternalProject_Add( external_boost
		PREFIX boost
		URL ${boost_URL}
        URL_HASH SHA1=${BOOST_SHA_CHECKSUM}
        BUILD_IN_SOURCE 1
        CONFIGURE_COMMAND ./bootstrap.sh
            --with-libraries=${BOOST_WITH_LIBRARIES}
            --prefix=<INSTALL_DIR>
        COMMAND ${CMAKE_COMMAND} -E remove <SOURCE_DIR>/tools/build/src/user-config.jam
        COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_BINARY_DIR}/user-config.jam <SOURCE_DIR>/tools/build/src/user-config.jam
        BUILD_COMMAND ./b2
        	-d+2
       		-j${NCPU}
       		--reconfigure
       		architecture=arm
       		address-model=32_64
        	toolset=darwin
        	target-os=iphone
        	macosx-version=iphone-${IOS_SDK_VERSION}
        	define=_LITTLE_ENDIAN
        	link=static
       		variant=release
        	install
        INSTALL_COMMAND ""
        INSTALL_DIR ${boost_INSTALL} )

set( BOOST_LIBRARY_SUFFIX .a )

# Expected output filenames for the boost libraries
set( BOOST_RESULT_LIBRARIES )
foreach( BOOST_COMPILE_LIBRARY ${BOOST_COMPILE_LIBRARIES} )
    set( LIBRARY_NAME libboost_${BOOST_COMPILE_LIBRARY}${BOOST_LIBRARY_SUFFIX} )
    list( APPEND BOOST_RESULT_LIBRARIES ${LIBRARY_NAME} )
endforeach( BOOST_COMPILE_LIBRARY )
add_dependencies( BOOST_COMPILE_LIBRARY external_boost )

# Check each static library for the included architectures and toss anything that shouldn't be included
foreach( BOOST_RESULT_LIBRARY ${BOOST_RESULT_LIBRARIES} )
    execute_process( COMMAND lipo -info ${boost_LIB_DIR}/${BOOST_RESULT_LIBRARY}
        COMMAND egrep "armv?[[:digit:]]+[[:lower:]]*" -o
        OUTPUT_VARIABLE ARCHIVE_LIBRARY_ARCHITECTURES )
    string(REGEX REPLACE "[\r\n]+" ";" ARCHIVE_LIBRARY_ARCHITECTURES ${ARCHIVE_LIBRARY_ARCHITECTURES} )
    set( REMOVE_ARCHITECTURES )
    foreach( ARCHIVE_LIBRARY_ARCHITECTURE ${ARCHIVE_LIBRARY_ARCHITECTURES} )
        list( FIND COMPILE_ARCHITECTURES ${ARCHIVE_LIBRARY_ARCHITECTURE} CONTAINS_ARCHITECTURE )
        if( NOT CONTAINS_ARCHITECTURE )
            list( APPEND REMOVE_ARCHITECTURES ${ARCHIVE_LIBRARY_ARCHITECTURE} )
        endif( NOT CONTAINS_ARCHITECTURE )
    endforeach( ARCHIVE_LIBRARY_ARCHITECTURE )
    foreach( REMOVE_ARCHITECTURE ${REMOVE_ARCHITECTURES} )
        execute_process( COMMAND lipo -remove ${REMOVE_ARCHITECTURE} -output ${BOOST_RESULT_LIBRARY} )
    endforeach( REMOVE_ARCHITECTURE )
endforeach( BOOST_RESULT_LIBRARY )

message( STATUS "${LIBRARY_ARCHITECTURES}" )


add_library( boost::date_time STATIC IMPORTED )
set_property( TARGET boost::date_time PROPERTY IMPORTED_LOCATION ${boost_LIB_DIR}/libboost_date_time${BOOST_LIBRARY_SUFFIX} )
set_property( TARGET boost::date_time PROPERTY INTERFACE_INCLUDE_DIRECTORIES ${boost_INCLUDE_DIR} )
add_dependencies( boost::date_time external_boost )

add_library( boost::system STATIC IMPORTED )
set_property( TARGET boost::system PROPERTY IMPORTED_LOCATION ${boost_LIB_DIR}/libboost_system${BOOST_LIBRARY_SUFFIX} )
set_property( TARGET boost::system PROPERTY INTERFACE_INCLUDE_DIRECTORIES ${boost_INCLUDE_DIR} )
add_dependencies( boost::system external_boost )

add_library( boost::filesystem STATIC IMPORTED )
set_property( TARGET boost::filesystem PROPERTY IMPORTED_LOCATION ${boost_LIB_DIR}/libboost_filesystem${BOOST_LIBRARY_SUFFIX} )
set_property( TARGET boost::filesystem PROPERTY INTERFACE_LINK_LIBRARIES boost::system )
set_property( TARGET boost::filesystem PROPERTY INTERFACE_INCLUDE_DIRECTORIES ${boost_INCLUDE_DIR} )
add_dependencies( boost::filesystem external_boost )