[Bug target/24012] [5/6/7 regression] #define _POSIX_C_SOURCE breaks #include <iostream>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24012

--- Comment #23 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 41068
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41068&action=edit
Use dynamic checks for C99 <wchar.h> functions

This fixes it and passes the testsuite on GNU/Linux and FreeBSD 11.0-RELEASE.

Even though this bug is a regression, this is too late in the gcc-7 process to
make a change like this. I'll commit it early for gcc-8 and backport it if it
doesn't cause any problems.