Clang (and GCC) always has __cplusplus >= C++17 now

...after 665ac8f3 "All supported versions of
Clang and GCC support at least C++17 now".  And even if __cplusplus might still
be < 201703L for old versions that only support -std=c++1z and not -std=c++17,
this #if should probably be enabled for those too.

(MSVC will first need adding /Zc:__cplusplus before it has a correct __cplusplus
value.)

Change-Id: If932b293d66e704357b10836839d221634bb1a3b
Reviewed-on: https://gerrit.libreoffice.org/64751
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

unotools/source/i18n/resmgr.cxx

@@ -21,7 +21,7 @@
 // workdir/UnpackedTarball/boost/boost/locale/format.hpp using "std::auto_ptr<data> d;", but must
 // come very early here in case <memory> is already (indirectly) included earlier:
 #include <config_libcxx.h>
-#if HAVE_LIBCXX && __cplusplus >= 201703L
+#if HAVE_LIBCXX
 #define _LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR
 #elif defined _MSC_VER && __cplusplus >= 201703L
 #define _HAS_AUTO_PTR_ETC 1