[Bug libstdc++/86513] ostringstream default constructor missing from libstdc++

[csaba_22 at yahoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86513

--- Comment #5 from Csaba Ráduly <csaba_22 at yahoo dot co.uk> ---
BTW, I wasn't building in the source directory.