libstdc++: [_GLIBCXX_DEBUG] Add backtrace generation on demand

   Add _GLIBCXX_DEBUG_BACKTRACE macro to activate backtrace generation 
on _GLIBCXX_DEBUG assertions. Prerequisite is to have configure the lib 
with:

   --enable-libstdcxx-backtrace=yes

   libstdc++-v3/ChangeLog:

   * include/debug/formatter.h
   [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_state): Declare.
   [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_create_state): Declare.
   [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_full_callback): Define.
   [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_error_callback): Define.
   [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_full_func): Define.
   [_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_full): Declare.
   [_GLIBCXX_HAVE_STACKTRACE](_Error_formatter::_M_backtrace_state): New.
   [_GLIBCXX_HAVE_STACKTRACE](_Error_formatter::_M_backtrace_full): New.
   * src/c++11/debug.cc (pretty_print): Rename into...
   (print_function): ...that.
   [_GLIBCXX_HAVE_STACKTRACE](print_backtrace): New.
   (_Error_formatter::_M_error()): Adapt.
   * src/libbacktrace/Makefile.am: Add backtrace.c.
   * src/libbacktrace/Makefile.in: Regenerate.
   * src/libbacktrace/backtrace-rename.h (backtrace_full): New.
   * testsuite/23_containers/vector/debug/assign4_neg.cc: Add backtrace
     generation.
   * doc/xml/manual/debug_mode.xml: Document _GLIBCXX_DEBUG_BACKTRACE.
   * doc/xml/manual/using.xml: Likewise.

Tested under Linux x86_64 normal and _GLIBCXX_DEBUG modes.

Ok to commit ?

François