fix freebsd build failure when DWT_CPP17_FILESYSTEM_IMPLEMENTATION=std

[NuLL3rr0r]

On FreeBSD I get:

[  2% 14/601] /usr/local/libexec/ccache/c++ -DBOOST_SPIRIT_THREADSAFE -DHAVE_GRAPHICSMAGICK -DHAVE_PDF_IMAGE -DWT_BUILDING -DWT_FONTSUPPORT_PANGO -DWT_WITH_OLD_INTERNALPATH_API -D_REENTRANT -Dwt_EXPORTS -I/home/mamadou/www/wt/work/.build -I/home/mamadou/www/wt/work/wt-4.12.1/src/web -I/home/mamadou/www/wt/work/wt-4.12.1/src -I/home/mamadou/www/wt/work/wt-4.12.1/src/Wt/Date/include -I/home/mamadou/www/wt/work/.build/src -I/home/mamadou/www/wt/work/wt-4.12.1/src/Wt/Dbo/backend/amalgamation -I/home/mamadou/www/wt/work/wt-4.12.1/src/thirdparty/qrcodegen -I/usr/local/include/GraphicsMagick -I/usr/local/include/pango-1.0 -I/usr/local/include/freetype2 -I/usr/local/include/harfbuzz -I/usr/local/include/glib-2.0 -I/usr/local/lib/glib-2.0/include -isystem /usr/local/include -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -O2 -pipe -fstack-protector-strong -fno-strict-aliasing  -DNDEBUG -std=c++20 -fPIC -fvisibility=hidden -fvisibility-inlines-hidden -pthread -MD -MT src/CMakeFiles/wt.dir/Wt/FlexLayoutImpl.C.o -MF src/CMakeFiles/wt.dir/Wt/FlexLayoutImpl.C.o.d -o src/CMakeFiles/wt.dir/Wt/FlexLayoutImpl.C.o -c /home/mamadou/www/wt/work/wt-4.12.1/src/Wt/FlexLayoutImpl.C
FAILED: [code=1] src/CMakeFiles/wt.dir/Wt/FlexLayoutImpl.C.o 
/usr/local/libexec/ccache/c++ -DBOOST_SPIRIT_THREADSAFE -DHAVE_GRAPHICSMAGICK -DHAVE_PDF_IMAGE -DWT_BUILDING -DWT_FONTSUPPORT_PANGO -DWT_WITH_OLD_INTERNALPATH_API -D_REENTRANT -Dwt_EXPORTS -I/home/mamadou/www/wt/work/.build -I/home/mamadou/www/wt/work/wt-4.12.1/src/web -I/home/mamadou/www/wt/work/wt-4.12.1/src -I/home/mamadou/www/wt/work/wt-4.12.1/src/Wt/Date/include -I/home/mamadou/www/wt/work/.build/src -I/home/mamadou/www/wt/work/wt-4.12.1/src/Wt/Dbo/backend/amalgamation -I/home/mamadou/www/wt/work/wt-4.12.1/src/thirdparty/qrcodegen -I/usr/local/include/GraphicsMagick -I/usr/local/include/pango-1.0 -I/usr/local/include/freetype2 -I/usr/local/include/harfbuzz -I/usr/local/include/glib-2.0 -I/usr/local/lib/glib-2.0/include -isystem /usr/local/include -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -O2 -pipe -fstack-protector-strong -fno-strict-aliasing  -DNDEBUG -std=c++20 -fPIC -fvisibility=hidden -fvisibility-inlines-hidden -pthread -MD -MT src/CMakeFiles/wt.dir/Wt/FlexLayoutImpl.C.o -MF src/CMakeFiles/wt.dir/Wt/FlexLayoutImpl.C.o.d -o src/CMakeFiles/wt.dir/Wt/FlexLayoutImpl.C.o -c /home/mamadou/www/wt/work/wt-4.12.1/src/Wt/FlexLayoutImpl.C
In file included from /home/mamadou/www/wt/work/wt-4.12.1/src/Wt/FlexLayoutImpl.C:7:
In file included from /home/mamadou/www/wt/work/wt-4.12.1/src/Wt/WApplication.h:26:
In file included from /home/mamadou/www/wt/work/wt-4.12.1/src/Wt/WEnvironment.h:17:
/home/mamadou/www/wt/work/wt-4.12.1/src/Wt/WLocale.h:139:39: error: no type named 'time_zone' in namespace 'std::chrono'
  139 |   void setTimeZone(const cpp20::date::time_zone *zone);
      |                          ~~~~~~~~~~~~~^
/home/mamadou/www/wt/work/wt-4.12.1/src/Wt/WLocale.h:145:22: error: no type named 'time_zone' in namespace 'std::chrono'
  145 |   const cpp20::date::time_zone *timeZone() const { return timeZone_; }
      |         ~~~~~~~~~~~~~^
/home/mamadou/www/wt/work/wt-4.12.1/src/Wt/WLocale.h:268:22: error: no type named 'time_zone' in namespace 'std::chrono'
  268 |   const cpp20::date::time_zone *timeZone_;
      |         ~~~~~~~~~~~~~^
In file included from /home/mamadou/www/wt/work/wt-4.12.1/src/Wt/FlexLayoutImpl.C:13:
In file included from /home/mamadou/www/wt/work/wt-4.12.1/src/Wt/StdGridLayoutImpl2.h:11:
/home/mamadou/www/wt/work/wt-4.12.1/src/Wt/StdLayoutImpl.h:82:16: warning: 'setObjectName' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
   82 |   virtual void setObjectName(const std::string& name);
      |                ^
/home/mamadou/www/wt/work/wt-4.12.1/src/Wt/WObject.h:183:16: note: overridden virtual function is here
  183 |   virtual void setObjectName(const std::string& name);
      |                ^
1 warning and 3 errors generated.
ninja: build stopped: subcommand failed.
===> Compilation failed unexpectedly.
Try to set MAKE_JOBS_UNSAFE=yes and rebuild before reporting the failure to
the maintainer.
*** Error code 1

Stop.
make[1]: stopped in /home/mamadou/www/wt
*** Error code 1

Stop.
make: stopped in /home/mamadou/www/wt
root@core:/home/mamadou/www/wt # nvim /home/mamadou/www/wt/work/wt-4.12.1/src/Wt/WLocale.h

It seems std::chrono additions in C++20 still not supported in libc++: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html

So, this fix is not just for FreeBSD, but also libc++.