Add --with-latest-c++ to explicitly opt in for -std=c++20/-std:c++latest

cd472d1d8489f30797f47d3f6dafede28c1feb90 "Compile as C++2a, where available" had
started to unconditionally check for support of -std=c++2a (and later also
-std=c++20) for Clang and GCC, but that can cause occasional issues especially
for Linux distros, see e.g. 55c724b93dfd4c9a1afb10d60fbc2d7a9a66cf61 "replace
boost::bimap in sdext pdfimport" or
<https://bugzilla.redhat.com/show_bug.cgi?id=1818723>
"/usr/include/boost/format/alt_sstream_impl.hpp incompatible with -std=c++20
(std::allocator::allocate hint argument)" (where
677c8de4fa79cd9b278b142013ba7f1c9e4e41c3 "external/boost: Adapt to
std::allocator parts removed in C++20" is not picked up due to
--with-system-boost).

So better require an explicit opt-in via a new --with-latest-c++.  And while at
it, also make that enable -std:c++latest for MSVC.

Change-Id: I2d1f03144fad9a7884562e56b1b76cab5eb8f080
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/92555
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/93204
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-by: Tor Lillqvist <tml@collabora.com>

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