This PR fixes the build when using recent Clang releases and libc++.
From https://github.com/hebasto/bitcoin-core-nightly/actions/runs/31400107892/job/93492660951:
[27/1645] Building CXX object qtbase/src/tools/syncqt/CMakeFiles/syncqt.dir/main.cpp.o
FAILED: [code=1] qtbase/src/tools/syncqt/CMakeFiles/syncqt.dir/main.cpp.o
/usr/bin/clang++-24 -stdlib=libc++ -DQT_EXPLICIT_QFILE_CONSTRUCTION_FROM_PATH -DQT_LEAN_HEADERS=1 -DQT_NAMESPACE=\"\" -DQT_NO_DEBUG -DQT_NO_FOREACH -DQT_NO_JAVA_STYLE_ITERATORS -DQT_NO_NARROWING_CONVERSIONS_IN_CONNECT -DQT_NO_QASCONST -DQT_NO_QEXCHANGE -DQT_NO_QSNPRINTF -DQT_USE_QSTRINGBUILDER -DQT_VERSION_MAJOR=6 -DQT_VERSION_MINOR=8 -DQT_VERSION_PATCH=4 -DQT_VERSION_STR=\"6.8.4\" -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-e34d706caeb/qtbase/src/tools/syncqt -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-e34d706caeb/qtbase/mkspecs/linux-clang -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-e34d706caeb/qtbase/include -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/x86_64-pc-linux-gnu/include -ffile-prefi
/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-e34d706caeb/qtbase/src/tools/syncqt/main.cpp:82:10: error: no member named 'transform' in namespace 'std'
82 | std::transform(s.begin(), s.end(), s.begin(),
| ^~~~~~~~~
/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-e34d706caeb/qtbase/src/tools/syncqt/main.cpp:89:10: error: no member named 'transform' in namespace 'std'
89 | std::transform(s.begin(), s.end(), s.begin(),
| ^~~~~~~~~
2 errors generated.
[28/1645] Building C object qtbase/src/3rdparty/pcre2/CMakeFiles/BundledPcre2.dir/src/pcre2_ucd.c.o
[29/1645] Building C object qtbase/src/3rdparty/pcre2/CMakeFiles/BundledPcre2.dir/src/pcre2_match.c.o
[30/1645] Building C object qtbase/src/3rdparty/pcre2/CMakeFiles/BundledPcre2.dir/src/pcre2_jit_compile.c.o
ninja: build stopped: subcommand failed.
gmake: *** [funcs.mk:343: /home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-e34d706caeb/./.stamp_built] Error 1
gmake: Leaving directory '/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends'
From https://github.com/hebasto/bitcoin-core-nightly/actions/runs/31403593334/job/93504547098:
[892/1645] Building CXX object qtbase/src/plugins/platforms/xcb/CMakeFiles/XcbQpaPrivate.dir/qxcbatom.cpp.o
FAILED: [code=1] qtbase/src/plugins/platforms/xcb/CMakeFiles/XcbQpaPrivate.dir/qxcbatom.cpp.o
/usr/bin/clang++-24 -stdlib=libc++ -DQT_ASCII_CAST_WARNINGS -DQT_BUILDING_QT -DQT_BUILD_XCB_PLUGIN -DQT_BUILD_XCB_QPA_LIB_LIB -DQT_CORE_LIB -DQT_DEPRECATED_WARNINGS -DQT_EXPLICIT_QFILE_CONSTRUCTION_FROM_PATH -DQT_GUI_LIB -DQT_LEAN_HEADERS=1 -DQT_MOC_COMPAT -DQT_NO_CAST_TO_ASCII -DQT_NO_DEBUG -DQT_NO_EXCEPTIONS -DQT_NO_FOREACH -DQT_NO_JAVA_STYLE_ITERATORS -DQT_NO_NARROWING_CONVERSIONS_IN_CONNECT -DQT_NO_QASCONST -DQT_NO_QEXCHANGE -DQT_NO_QSNPRINTF -DQT_USE_QSTRINGBUILDER -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/src/plugins/platforms/xcb/XcbQpaPrivate_autogen/include -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/src/plugins/platforms/xcb -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/src/plugins/platforms/xcb/gl_integrations -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/src/3rdparty/xcb/include -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/src/corelib -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/include/QtCore/6.8.4 -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/include/QtCore/6.8.4/QtCore -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/include -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/include/QtCore -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/mkspecs/linux-clang -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/src/gui -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/include/QtGui/6.8.4 -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/include/QtGui/6.8.4/QtGui -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/include/QtGui -I/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/x86_64-pc-linux-gnu/include -ffile-prefix-map=/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777=/usr -O2 -std=gnu++20 -fPIC -fvisibility=hidden -fvisibility-inlines-hidden -Wall -Wextra -fno-exceptions -ftemplate-depth=1024 -ffunction-sections -fdata-sections -U_FORTIFY_SOURCE -fcf-protection=full -D_FORTIFY_SOURCE=3 -ftrivial-auto-var-init=pattern -fstack-protector-strong -MD -MT qtbase/src/plugins/platforms/xcb/CMakeFiles/XcbQpaPrivate.dir/qxcbatom.cpp.o -MF qtbase/src/plugins/platforms/xcb/CMakeFiles/XcbQpaPrivate.dir/qxcbatom.cpp.o.d -o qtbase/src/plugins/platforms/xcb/CMakeFiles/XcbQpaPrivate.dir/qxcbatom.cpp.o -c /home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/src/plugins/platforms/xcb/qxcbatom.cpp
/home/runner/work/bitcoin-core-nightly/bitcoin-core-nightly/depends/work/build/x86_64-pc-linux-gnu/qt/6.8.4-d5ec21e4777/qtbase/src/plugins/platforms/xcb/qxcbatom.cpp:237:13: error: use of undeclared identifier 'free'
237 | free(reply);
| ^~~~
1 error generated.
