Make --with-latest-c++ support future C++26

...as already supported by
<https://github.com/llvm/llvm-project/commit/b763d6a4ed4650c74c6846d743156468563b0e31>
"Add C++26 compile flags."

Change-Id: I0e7b55138222c0f0a81d4aec9a20da7922c51df2
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/151825
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

configure.ac

@@ -7529,7 +7529,7 @@ if test -z "${CXXFLAGS_CXX11+x}"; then
     elif test "$GCC" = "yes" -o "$COM_IS_CLANG" = TRUE; then
         my_flags='-std=c++17 -std=c++1z'
         if test "$with_latest_c__" = yes; then
-            my_flags="-std=c++23 -std=c++2b -std=c++20 -std=c++2a $my_flags"
+            my_flags="-std=c++26 -std=c++2c -std=c++23 -std=c++2b -std=c++20 -std=c++2a $my_flags"
         fi
         for flag in $my_flags; do
             if test "$COM" = MSC; then