[MacPorts] #64620: gcc10 @10.3.0_1+universal fails to build on 10.13

[MacPorts]

#64620: gcc10 @10.3.0_1+universal fails to build on 10.13
------------------------+----------------------
  Reporter:  JohnFHall  |      Owner:  (none)
      Type:  defect     |     Status:  assigned
  Priority:  Normal     |  Milestone:
 Component:  ports      |    Version:  2.7.1
Resolution:             |   Keywords:
      Port:  gcc10      |
------------------------+----------------------
Description changed by ryandesign:

Old description:

> :info:build /opt/local/var/macports/build
>
> /_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_lang_gcc10/gcc10/work/build/x86_64
> -apple-darwin17/libstdc++-v3/include/bits/basic_string.h:48:11: fatal
> error: string_view: No such file or directory
>
> :info:build    48 | # include <string_view>
>
> :info:build       |           ^~~~~~~~~~~~~
>
> :info:build compilation terminated.

New description:

 {{{
 :info:build /opt/local/var/macports/build
 /_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_lang_gcc10/gcc10/work/build/x86_64
 -apple-darwin17/libstdc++-v3/include/bits/basic_string.h:48:11: fatal
 error: string_view: No such file or directory
 :info:build    48 | # include <string_view>
 :info:build       |           ^~~~~~~~~~~~~
 :info:build compilation terminated.}}}

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Ticket URL: <https://trac.macports.org/ticket/64620#comment:3>
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