lpython needs filesystem on < 10.15 when Clang is used

[barracuda156]

[  9%] Building CXX object src/libasr/CMakeFiles/asr.dir/codegen/asr_to_julia.cpp.o
cd /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lpython/lpython/work/build/src/libasr && /usr/bin/clang++ -DHAVE_WHEREAMI=1 -DHAVE_ZLIB=1 -DLCOMPILERS_FAST_ALLOC=1 -I/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lpython/lpython/work/build/src/libasr/.. -I/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lpython/lpython/work/lpython-0.20.0/src/libasr/.. -pipe -Os -DNDEBUG -I/opt/local/include -stdlib=libc++ -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk -Wall -Wextra -O3 -march=native -funroll-loops -DNDEBUG -arch x86_64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk -mmacosx-version-min=10.14 -fPIC -std=gnu++17 -MD -MT src/libasr/CMakeFiles/asr.dir/codegen/asr_to_julia.cpp.o -MF CMakeFiles/asr.dir/codegen/asr_to_julia.cpp.o.d -o CMakeFiles/asr.dir/codegen/asr_to_julia.cpp.o -c /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lpython/lpython/work/lpython-0.20.0/src/libasr/codegen/asr_to_julia.cpp
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lpython/lpython/work/lpython-0.20.0/src/libasr/codegen/asr_to_cpp.cpp:6:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lpython/lpython/work/lpython-0.20.0/src/libasr/../libasr/codegen/asr_to_cpp.h:5:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lpython/lpython/work/lpython-0.20.0/src/libasr/../libasr/utils.h:6:10: fatal error: 'filesystem' file not found
#include <filesystem>
         ^~~~~~~~~~~~
1 error generated.