libgcc @6.1.0_0 building fails

[lpn25]

Building fails when upgrading libgcc from 5.3.0_1 to 6.1.0_0. This is on OS X 10.10.5; with Xcode 7.2.1 (the newer 7.3.1 doesn't seem to install on Yosemite)

This is what it shows in

/opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_lang_gcc6/libgcc/main.log

just before the first error:

:info:build Comparing stages 2 and 3
:info:build warning: gcc/cc1-checksum.o differs
:info:build warning: gcc/cc1obj-checksum.o differs
:info:build warning: gcc/cc1objplus-checksum.o differs
:info:build warning: gcc/cc1plus-checksum.o differs
:info:build Bootstrap comparison failure!
:info:build gcc/ipa-icf.o differs
:info:build gcc/tree-ssa-structalias.o differs
:info:build libcpp/expr.o differs
:info:build make[2]: *** [compare] Error 1
:info:build make[2]: Leaving directory `/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.or$
:info:build make[1]: *** [stage3-bubble] Error 2
:info:build make[1]: Leaving directory `/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.or$
:info:build make: *** [bootstrap] Error 2
:info:build make: Leaving directory `/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_r$
:info:build Command failed:  cd "/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_relea$
:info:build Exit code: 2
:error:build org.macports.build for port libgcc returned: command execution failed
:debug:build Error code: CHILDSTATUS 478 2
:debug:build Backtrace: command execution failed

[dbevans (David B. Evans)]

See ongoing discussion in #51245.