[Bug pch/68176] [4.9/5/6 Regression] all pch tests fail on eglibc systems (with bits/predefs.h)

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68176

--- Comment #6 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
The original concept for stdc-predef.h includes that it might include 
other headers - in particular, that it might #include_next 
<stdc-predef.h>, so that e.g. libdfp could provide its own stdc-predef.h 
that defines relevant __STDC_* macros to say that DFP library 
functionality is available before including the glibc version, when you 
use -isystem /usr/include/dfp.

Now, libdfp currently doesn't do that, and such use of libdfp might not be 
expected to work with PCH, so fixincluding may well be sufficient for this 
bug at present.