x11/waybar: unbreak build with libc++ 17

In file included from ../src/modules/clock.cpp:1:
In file included from ../include/modules/clock.hpp:4:
In file included from ../include/util/date.hpp:29:
/usr/local/include/date/tz.h:235:8: error: use of overloaded operator '<<' is ambiguous (with operand types 'basic_ostream<char, char_traits<char>>' and 'const sys_seconds' (aka 'const time_point<std::chrono::system_clock, std::chrono::duration<long long, std::ratio<1, 1>>>'))

PR:		275651
Reported by:	pkg-fallout

x11/waybar/Makefile

@@ -160,4 +160,25 @@ pre-test:
 		${TOUCH} ${WRKDIR}/.meson_build_tests; \
 	fi
 
+# XXX https://github.com/HowardHinnant/date/issues/799
+.if ${CXX} == c++ && exists(/usr/include/c++/v1/__chrono/concepts.h)
+BUILD_DEPENDS+=	${CMAKE_BIN}:devel/cmake-core
+CONFIGURE_ENV+=	CC="${CC}" CXX="${CXX}" CPP="${CPP}"
+CMAKE_BIN=	${LOCALBASE}/bin/cmake
+
+# XXX Move into separate port and standardize via USES
+GH_TUPLE+=	llvm:llvm-project:llvmorg-16.0.1:libcxx
+CXXFLAGS+=	-nostdinc++ -isystem${WRKDIR}/libcxx_prefix/include/c++/v1
+# Don't link against new libc++ as it's not necessary
+#LDFLAGS+=	-nostdlib++ -L${WRKDIR}/libcxx_prefix/lib -l:libc++.a -lcxxrt
+
+pre-configure:	bundled-libcxx
+bundled-libcxx:
+	@${SETENV} ${CONFIGURE_ENV} ${CMAKE_BIN} ${CMAKE_ARGS} \
+		-DLIBCXX_INCLUDE_BENCHMARKS:BOOL=OFF \
+		-DCMAKE_INSTALL_PREFIX:PATH=${WRKDIR}/libcxx_prefix \
+		-B ${WRKDIR}/libcxx_build -S ${WRKSRC_libcxx}/libcxx
+	@${DO_MAKE_BUILD:NDESTDIR*} install -C ${WRKDIR}/libcxx_build
+.endif # exists(/usr/include/c++/v1/__chrono/concepts.h)
+
 .include <bsd.port.mk>

x11/waybar/distinfo

@@ -1,3 +1,5 @@
 TIMESTAMP = 1699016827
 SHA256 (Alexays-Waybar-0.9.24_GH0.tar.gz) = 57aa7860bc066ebf4f3327dafa9841100b098c0dec1dce4baaa1fae63e9b57ae
 SIZE (Alexays-Waybar-0.9.24_GH0.tar.gz) = 275416
+SHA256 (llvm-llvm-project-llvmorg-16.0.1_GH0.tar.gz) = b5a9ff1793b1e2d388a3819bf35797002b1d2e40bb35a10c65605e0ea1435271
+SIZE (llvm-llvm-project-llvmorg-16.0.1_GH0.tar.gz) = 179289803