gdb を起動すると、次のようなメッセージが表示されます。
$ gdb a.out
GNU gdb 6.3.50-20050815 (Apple version gdb-1708) (Thu Nov 3 21:59:02 UTC 2011)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin"...Reading symbols for shared libraries .
warning: Could not find object file "/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_lang_gcc47/gcc47/work/build/x86_64-apple-darwin11/libstdc++-v3/src/../libsupc++/.libs/libsupc++convenience.a(array_type_info.o)" - no debug information available for "../../../../gcc-4.7.0/libstdc++-v3/libsupc++/array_type_info.cc".
warning: Could not find object file "/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_lang_gcc47/gcc47/work/build/x86_64-apple-darwin11/libstdc++-v3/src/../libsupc++/.libs/libsupc++convenience.a(bad_alloc.o)" - no debug information available for "../../../../gcc-4.7.0/libstdc++-v3/libsupc++/bad_alloc.cc".
libstdc++ を再度フェッチする必要があるようですが、macports ではどうですか?