This resolves a longstanding issue where the lock policy for shared_ptr
reference counting depends on compilation options when the header is
included, so that different -march options can cause ABI changes. For
example, objects compiled with -march=armv7 will use atomics to
synchronize reference counts, and objects compiled with -march=armv5t
will use a mutex. That means the shared_ptr control block will have a
different layout in different objects, causing ODR violations and
undefined behaviour. This was the root cause of PR libstdc++/42734 as
well as PR libstdc++/67843.

The solution is to decide on the lock policy at build time, when
libstdc++ is configured. The configure script checks for the
availability of the necessary atomic built-ins for the target and fixes
that choice permanently. Different -march flags used to compile user
code will not cause changes to the lock policy. This results in an ABI
change for certain compilations, but only where there was already an ABI
incompatibility between the libstdc++.so library and objects built with
an incompatible -march option. In general, this means a more stable ABI
that isn't silently altered when -march flags make addition atomic ops
available.

To force a target to use "atomic" or "mutex" the new configure option
--with-libstdcxx-lock-policy can be used.

In order to turn ODR violations into linker errors, the uses of
shared_ptr in filesystem directory iterators have been replaced
with __shared_ptr, and explicit instantiations are declared. This
ensures that object files using those types cannot link to libstdc++
libs unless they use the same lock policy.

	PR libstdc++/67843
	* acinclude.m4 (GLIBCXX_ENABLE_LOCK_POLICY): Add new macro
	that defines _GLIBCXX_HAVE_ATOMIC_LOCK_POLICY.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Use GLIBCXX_ENABLE_LOCK_POLICY.
	* doc/xml/manual/configure.xml: Document new configure option.
	* include/bits/fs_dir.h (directory_iterator): Use __shared_ptr
	instead of shared_ptr.
	(recursive_directory_iterator): Likewise.
	(__shared_ptr<_Dir>): Add explicit instantiation declaration.
	(__shared_ptr<recursive_directory_iterator::_Dir_stack>): Likewise.
	* include/bits/shared_ptr_base.h (__allocate_shared, __make_shared):
	Add default template argument for _Lock_policy template parameter.
	* include/ext/concurrence.h (__default_lock_policy): Check macro
	_GLIBCXX_HAVE_ATOMIC_LOCK_POLICY instead of checking if the current
	target supports the builtins for compare-and-swap.
	* src/filesystem/std-dir.cc (__shared_ptr<_Dir>): Add explicit
	instantiation definition.
	(__shared_ptr<recursive_directory_iterator::_Dir_stack>): Likewise.
	(directory_iterator, recursive_directory_iterator): Use __make_shared
	instead of make_shared.

From-SVN: r266533