mdtree-cpp needs filesystem support in compiler

[barracuda156]

[ 50%] Building CXX object CMakeFiles/mdtree.dir/mdtree.cpp.o
/usr/bin/clang++   -pipe -Os -DNDEBUG -I/opt/local/include -stdlib=libc++ -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk -std=gnu++2a -arch x86_64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk -mmacosx-version-min=10.14 -MD -MT CMakeFiles/mdtree.dir/mdtree.cpp.o -MF CMakeFiles/mdtree.dir/mdtree.cpp.o.d -o CMakeFiles/mdtree.dir/mdtree.cpp.o -c /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_textproc_mdtree-cpp/mdtree-cpp/work/1.0.0/mdtree.cpp
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_textproc_mdtree-cpp/mdtree-cpp/work/1.0.0/mdtree.cpp:1:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_textproc_mdtree-cpp/mdtree-cpp/work/1.0.0/mdtree.h:11:10: fatal error: 'filesystem' file not found
#include <filesystem>
         ^~~~~~~~~~~~
1 error generated.

I have nothing to test this locally on (modern gcc supports filesystem out of the box), but looks like it needs legacysupport help here :)