gcc13: scope -std=gnu++17 to the target branch (char8_t fixed; probe still open)

libcody is compiled by the CROSS compiler, not the host one -- the build
log shows
    x86_64-unknown-redox-g++ ... -c -o buffer.o .../libcody/buffer.cc
because --host is the Redox target. So plain CXXFLAGS is the right knob,
not CXXFLAGS_FOR_BUILD as the previous commit assumed. Scoped to the
gcc-native branch only.

Verified: char8_t errors go 100+ -> 0.

STILL OPEN. With CXXFLAGS set, the top-level configure now fails its
build-side probe:
    checking whether gcc accepts -g... no
    configure: error: C++11 is required
The two states are exclusive as things stand:
  - CXXFLAGS carrying -std=gnu++17  -> char8_t fixed, C++11 probe fails
  - CXXFLAGS without it             -> probe passes, char8_t fails
so something in the exported CXXFLAGS is unacceptable to the HOST gcc
that configure probes (the cookbook adds -fno-hardened -DHAVE_ALLOCA_H=1
-fPIC alongside a target -I). The recipe unsets CFLAGS/CPPFLAGS/LDFLAGS
for this branch but leaves CXXFLAGS set, which is the likely culprit --
the next step is to give the build-side compiler its own clean flags
rather than inheriting the target's.

recipes/dev/gcc13/recipe.toml

@@ -42,15 +42,7 @@ export gcc_cv_sys_sdt_h=no
 # in libcody/{buffer,client,server}.cc.
 #
 # This is not a workaround: C++17 is the dialect this release was written
-# against, and GCC's own build documentation expects the host compiler to be
-# driven at the standard the sources use. Applies to both the cross-toolchain
-# and the gcc-native target build below, which share this script.
-# Build-side ONLY. libcody is compiled by CXX_FOR_BUILD (the host g++ 16),
-# which is where the char8_t failures occur. Putting -std=gnu++17 into plain
-# CXXFLAGS also hands it to the Redox cross g++, whose C++ probe then fails
-# during the freestanding stage before libstdc++ exists:
-#     configure: error: C++11 is required
-export CXXFLAGS_FOR_BUILD="${CXXFLAGS_FOR_BUILD:+$CXXFLAGS_FOR_BUILD }-std=gnu++17"
+# against. Applied in the target branch below, not here -- see the note there.
 
 if [ "${COOKBOOK_HOST_SYSROOT}" = "/usr" ]; then
 # not specifying --enable-shared as it will link shared deps
@@ -68,6 +60,17 @@ COOKBOOK_CONFIGURE_FLAGS=(
 else
 export CFLAGS_FOR_TARGET="${CPPFLAGS}" CXXFLAGS_FOR_TARGET="${CPPFLAGS}" LDFLAGS_FOR_TARGET="${LDFLAGS}"
 export CC_FOR_BUILD="$CC_WRAPPER gcc" CXX_FOR_BUILD="$CC_WRAPPER g++"
+# GCC 13's sources are C++17. libcody is compiled here by the CROSS compiler --
+# the build log shows
+#     x86_64-unknown-redox-g++ ... -c -o buffer.o .../libcody/buffer.cc
+# because --host is the Redox target -- and that compiler is now GCC 16, which
+# defaults to C++20. C++20 changed u8"" literals from const char[] to
+# const char8_t[], which libcody uses throughout, so the build collapses with
+#     error: invalid conversion from 'const char8_t*' to 'const char*'
+# Scoped to this branch: the freestanding cross-toolchain branch above must not
+# get it, or its C++ probe fails before libstdc++ exists with
+#     configure: error: C++11 is required
+export CXXFLAGS="${CXXFLAGS:+$CXXFLAGS }-std=gnu++17"
 unset CFLAGS CPPFLAGS LDFLAGS
 COOKBOOK_CONFIGURE_FLAGS+=(
     --with-sysroot=/