...at least from "g++ (GCC) 5.1.1 20150618 (Red Hat 5.1.1-4)" with
--disable-debug, when a namespace-scope const variable with a "complex"
initializer declared in an include file remains unused.

Avoid that warning via SAL_CONSTEXPR, which in turn requires large parts of
o3tl::is_typed_flags to be SAL_CONSTEXPR, which in turn requires a new
HAVE_CXX14_CONSTEXPR to allow assert in constexpr functions, which in turn
requires using -std=c++14 instead of -std=c++11 where available, which in turn
(a) requires to /not/ use -std=c++14 if it would run into a bug between Clang
and libstdc++ discussed at <https://llvm.org/bugs/show_bug.cgi?id=24115>
"llvm-nm fails to build with gcc 5.1's libstdc++" (and which hits us in
sfx2/source/control/thumbnailview.cxx), and (b) requires a new
HAVE_CXX14_SIZED_DEALLOCATION to work around GCC 5.1 -Werror=sized-deallocation
(where Clang >= 3.7 only supports C++14 sized deallocation when explictly
enabled via -fsized-deallocation, btw).

This effectively reverts ff6462e6 "avoid unused
variable warning:" again.

Change-Id: I424e3561452a3e6d8c8a9604d6c737cab49840c4
Reviewed-on: https://gerrit.libreoffice.org/18918Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>