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

Adaptation of this change to this branch. The point is to avoid
compiling as C++2a as the code for externals has not been patched
properly for that here. Here is the original commit message even if I
assume it is a bit misleading in this branch:

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>
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/94197
Tested-by: Tor Lillqvist <tml@collabora.com>

1 files changed, 16 insertions, 2 deletions

diff --git a/configure.ac b/configure.ac
index 18451469f752..7c3281999772 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2149,6 +2149,12 @@ libo_FUZZ_ARG_WITH(lxml,
          report widget classes and ids.]),
 ,)
 
+libo_FUZZ_ARG_WITH(latest-c++,
+    AS_HELP_STRING([--with-latest-c++],
+        [Try to enable the latest features of the C++ compiler, even if they are not yet part of a
+         published standard.]),,
+        [$with_latest_c__=no])
+
 dnl ===================================================================
 dnl Branding
 dnl ===================================================================
@@ -6462,9 +6468,17 @@ dnl ===================================================================
 AC_MSG_CHECKING([whether $CXX_BASE supports C++17])
 CXXFLAGS_CXX11=
 if test "$COM" = MSC -a "$COM_IS_CLANG" != TRUE; then
-    CXXFLAGS_CXX11='-std:c++17 -Zc:__cplusplus'
+    if test "$with_latest_c__" = yes; then
+        CXXFLAGS_CXX11=-std:c++latest
+    else
+        CXXFLAGS_CXX11=-std:c++17
+    fi
+    CXXFLAGS_CXX11="$CXXFLAGS_CXX11 -Zc:__cplusplus"
 elif test "$GCC" = "yes" -o "$COM_IS_CLANG" = TRUE; then
-    my_flags='-std=c++2a -std=c++17 -std=c++1z'
+    my_flags='-std=c++17 -std=c++1z'
+    if test "$with_latest_c__" = yes; then
+        my_flags="-std=gnu++2a -std=c++2a $my_flags"
+    fi
     for flag in $my_flags; do
         if test "$COM" = MSC; then
             flag="-Xclang $flag"