...as demanded by the C++17 standard, and required in
sc/source/core/tool/math.cxx.  At least when building an --arch=i386 Flatpak
(where CFLAGS/CXXFLAGS are passed in containing, among others, -O2), that failed
when using -std=gnu++2a (instead of -std=c++2a) due to what looks like an error
in glibc (cf. <https://flathub.org/builds/#/builders/22/builds/797>).

...and fix fallout in external/firebird for Linux x86 32-bit build:  While GCC
defines both __i386 and __i386__ (both as 1) regardless of -std=gnu++* vs.
-std=c++*, it defines i386 (as 1) only for -std=gnu++*.  But various places in
the external/firebird sources check for i386 (among them src/common/common.h,
which fails with

  #error Define FB_CPU for your platform

if i386 is not defined).

Change-Id: I7dce1961c79aeaccc82b1e2bdc350e02730d46af
Reviewed-on: https://gerrit.libreoffice.org/67105
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>