Boost >= v1.72.0 breaks with ICU_LINK specified

[evoskuil]

Boost applied a breaking change to their build process.

As of boost 1.72.0 the ICU_LINK symbol is no longer supported and produces a hard stop. See tags here: boostorg/regex@fc4dc17 and discussion here boostorg/regex#89

[evoskuil]

This job succeeded, with no difference in ICU_LINK config, all others failed (1 failed due to non-preinstall and non-discovery).

The difference appears to be that in this boost build, WITH_ICU is not set.

      <job system="linux" compiler="gcc" link="dynamic" version="4.8" sudo="false" >
        <option value="--disable-static" />
        <option value="--build-boost" />
        <option value="--prefix=$TRAVIS_BUILD_DIR/my-prefix" />
        <option value="CFLAGS='-Os -s'" />
        <option value="CXXFLAGS='-Os -s'" />
        <!--
        https://travis-ci.org/github/libbitcoin/libbitcoin-system/jobs/761563137
        gcc (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609
        --------------------------------------------------------------------
        OS                    : Linux
        PARALLEL              : 2
        CC                    : gcc-4.8
        CXX                   : g++-4.8
        CPPFLAGS              : 
        CFLAGS                : 
        CXXFLAGS              : 
        LDFLAGS               : 
        LDLIBS                : 
        WITH_ICU              : 
        WITH_PNG              : 
        WITH_QRENCODE         : 
        BUILD_ICU             : 
        BUILD_ZLIB            : 
        BUILD_PNG             : 
        BUILD_QRENCODE        : 
        BUILD_BOOST           : yes (clean 1.72.0)
        BUILD_DIR             : build-libbitcoin-system
        PREFIX                : /home/travis/build/libbitcoin/libbitcoin-system/my-prefix
        DISABLE_SHARED        : 
        DISABLE_STATIC        : yes
        with_boost            : --with-boost=/home/travis/build/libbitcoin/libbitcoin-system/my-prefix
        with_pkgconfigdir     : --with-pkgconfigdir=/home/travis/build/libbitcoin/libbitcoin-system/my-prefix/lib/pkgconfig
        --------------------------------------------------------------------
        Libbitcoin boost configuration.
        --------------------------------------------------------------------
        variant               : release
        threading             : multi
        toolset               : gcc-4.8
        cxxflags              : 
        linkflags             : 
        link                  : shared
        boost.locale.iconv    : on
        boost.locale.posix    : on
        -sNO_BZIP2            : 1
        -sICU_PATH            : 
        -sICU_LINK            : 
        -sZLIB_LIBPATH        : /home/travis/build/libbitcoin/libbitcoin-system/my-prefix/lib
        -sZLIB_INCLUDE        : /home/travis/build/libbitcoin/libbitcoin-system/my-prefix/include
        -j                    : 2
        -d0                   : [supress informational messages]
        -q                    : [stop at the first error]
        --reconfigure         : [ignore cached configuration]
        --prefix              : /home/travis/build/libbitcoin/libbitcoin-system/my-prefix
        BOOST_OPTIONS         : --with-atomic --with-chrono --with-date_time --with-filesystem --with-iostreams --with-locale --with-log --with-program_options --with-regex --with-system --with-thread --with-test
        --------------------------------------------------------------------
        Clean libbitcoin and test builds.
        -->
      </job>

[evoskuil]

Summary matrix for travis at https://travis-ci.org/github/libbitcoin/libbitcoin-system/builds/761563132

clang static   failure (WITH_ICU)
clang dynamic  failure (WITH_ICU)
gcc   static   failure (WITH_ICU)
gcc   dynamic  success
osx   static   failure (WITH_ICU)
osx   dynamic  undetected

[evoskuil]

Resolved in gsl.install.sh in libbitcoin-build by removal of the ICU_LINK flag definition.