libstdc++: Fix libbacktrace build files

libstdc++: Fix libbacktrace build files

This makes it possible to combine --enable-libstdcxx-debug with
--enable-libstdcxx-backtrace, by adding a rule to src/Makefile to copy
the backtrace-supported.h header into the src/debug/libbacktrace
directory.

Add libbacktrace path to testsuite flags so the tests can link without
having the library installed.

Also fix some warnings when running automake for the libbacktrace
makefile.

Use a per-library CPPFLAGS variable to fix:

src/libbacktrace/Makefile.am:38: warning: AM_CPPFLAGS multiply defined in condition TRUE ...
fragment.am:43: ... 'AM_CPPFLAGS' previously defined here
src/libbacktrace/Makefile.am:32:   'fragment.am' included from here

Create symlinks to the libbacktrace sources to fix:

src/libbacktrace/Makefile.am:55: warning: source file '../../../libbacktrace/atomic.c' is in a subdirectory,
src/libbacktrace/Makefile.am:55: but option 'subdir-objects' is disabled

libstdc++-v3/ChangeLog:

	* scripts/testsuite_flags.in: Add src/libbacktrace/.libs to
	linker search paths.
	* src/Makefile.am: Fix src/debug/libbacktrace build.
	* src/Makefile.in: Regenerate.
	* src/libbacktrace/Makefile.am: Use per-library CPPFLAGS
	variable. Use symlinks for the source files.
	* src/libbacktrace/Makefile.in: Regenerate.