[Bug bootstrap/116730] build is broken when building in subdirectory of a subdirectory of the src directory

[schwab--- via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116730

--- Comment #15 from Andreas Schwab <sch...@linux-m68k.org> ---
Does it help to change the symlink creation rules in
libstdc++-v3/src/libbacktrace/Makefile.am to use $(top_srcdir)/.. instead of
../../..?  I think that should prevent VAPTH from interfering.

%.c: $(top_srcdir)/../libbacktrace/%.c
        $(LN_S) $< $@

cp-demangle.c: $(top_srcdir)/../libiberty/cp-demangle.c
        $(LN_S) $< $@