[MacPorts] #55005: GMP fails to build on Sierra with Xcode 9.0

[MacPorts]

#55005: GMP fails to build on Sierra with Xcode 9.0
-------------------------+-------------------
 Reporter:  lucaspettey  |      Owner:
     Type:  defect       |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:  2.4.1
 Keywords:               |       Port:  gmp
-------------------------+-------------------
 The following message is received when trying to build gmp on Sierra with
 Xcode 9.0:

 --->  Configuring gmp
 Error: Failed to configure gmp, consult
 /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_devel_gmp/gmp/work/gmp-6.1.2/config.log

 I will attach the whole log for reference, but the last lines of the log
 file indicate an issue with clang++:

 :info:configure checking whether we are using the GNU C++ compiler... yes
 :info:configure checking whether /usr/bin/clang++ -stdlib=libc++ accepts
 -g... yes
 :info:configure checking C++ compiler /usr/bin/clang++ -stdlib=libc++
 -I/opt/local/include -O2 -pedantic -fomit-frame-pointer -m64
 -mtune=sandybridge -march=sandybridge... no, std iostream
 :info:configure checking C++ compiler /usr/bin/clang++ -stdlib=libc++
 -I/opt/local/include -g -O2... no, std iostream
 :info:configure configure: error: C++ compiler not available, see
 config.log for details
 :info:configure Command failed:  cd
 "/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_devel_gmp/gmp/work/gmp-6.1.2"
 && ./configure --prefix=/opt/local --enable-cxx
 :info:configure Exit code: 1

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Ticket URL: <https://trac.macports.org/ticket/55005>
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