[Bug bootstrap/112534] [14 regression] build failure after r14-5424-gdb50aea6259545 using gcc 4.8.5

["arsen at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112534

--- Comment #6 from Arsen Arsenović <arsen at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #5)
> The problem is that the toplevel configure (which is autoconf 2.69 as pretty
> much everything in gcc) uses the older AC_PROG_CC, which only checks for
> -std=gnu99 -std=c99 -c99 -AC99 -D_STDC_C99= -qlanglvl=extc99, not for
> -std=gnu11.
> And sets
> CC = @CC@
> in toplevel Makefile.in to
> CC = gcc -std=gnu99
> in toplevel objdir Makefile.  That gets then passed to in-tree gettext (if
> present, I really don't think you need it on powerpc64-linux-gnu, perhaps
> download_prerequisities should be smarter and check if gettext is really
> needed) configure (where it just means
> CC is set there to gcc -std=gnu99 -std=gnu11 in gettext/Makefile), but worse
> is passed as CC="gcc -std=gnu99" in environment down when doing make all in
> the gettext subdir.
> I think that is something very similar to how CXX="g++ -std=c++11" is being
> passed down
> to in-tree isl build and breaks with recent isl which wants to use C++17 or
> what.
> Strangely, in my x86_64-linux toplevel Makefile I only have
> CC = gcc
> CXX = g++ -std=c++11
> Dunno why it hasn't added -std=gnu99 there, maybe because that gcc already
> defaults to gnu17?  Anyway, even when CC = gcc, I think that is passed down
> to make of the in-tree compilations.
> So, I guess if we don't want to switch to autoconf 2.70 or later (which I
> think is a lot of work), one possibility if we know gettext relies on C11
> and newest ISL relies on C++17 (does it really?) would be to add explicit
> probing in configure for -std=gnu11 or -std=c11 and if that works, pass it
> down in the gettext build case; and similarly for isl.  Maybe better not in
> CC/CXX but in CFLAGS/CXXFLAGS?
> The propagation of flags is done in $(HOST_EXPORTS) and for stage2+
> $(POSTSTAGE1_HOST_EXPORTS).
> 
> Looking around in Makefile.def, I see e.g. for gmp/mpfr we use
>                 extra_make_flags='AM_CFLAGS="-DNO_ASM"';
> and for isl
>                 extra_make_flags='V=1';
> Dunno if it would work to add for the gettext case
>                 extra_make_flags='CFLAGS="$(CFLAGS) @C11_CFLAGS@"';
> with configure check for C11_FLAGS or something similar.

yes, that seems doable but I am curious about why the flag propagation via
export is necessary.  would each configure not already have the appropriate
flags set?