lfortran is broken on <= 10.14 with Clang due to missing filesystem

[barracuda156]

​https://ports.macports.org/port/lfortran/details

Example from Mojave:

[ 11%] Building CXX object src/libasr/CMakeFiles/asr.dir/codegen/asr_to_wasm.cpp.o
cd /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/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_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/.. -pipe -Os -DNDEBUG -I/opt/local/include -stdlib=libc++ -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk -Wall -Wextra -O3 -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_wasm.cpp.o -MF CMakeFiles/asr.dir/codegen/asr_to_wasm.cpp.o.d -o CMakeFiles/asr.dir/codegen/asr_to_wasm.cpp.o -c /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/codegen/asr_to_wasm.cpp
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/codegen/asr_to_julia.cpp:2:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/asr_utils.h:12:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/utils.h:6:10: fatal error: 'filesystem' file not found
#include <filesystem>
         ^~~~~~~~~~~~
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/codegen/asr_to_c.cpp:6:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/codegen/asr_to_c.h:5:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/utils.h:6:10: fatal error: 'filesystem' file not found
#include <filesystem>
         ^~~~~~~~~~~~
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/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_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/codegen/asr_to_cpp.h:5:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/utils.h:6:10: fatal error: 'filesystem' file not found
#include <filesystem>
         ^~~~~~~~~~~~
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/codegen/asr_to_py.cpp:6:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/asr_utils.h:12:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/utils.h:6:10: fatal error: 'filesystem' file not found
#include <filesystem>
         ^~~~~~~~~~~~
1 error generated.
make[2]: *** [src/libasr/CMakeFiles/asr.dir/codegen/asr_to_py.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
1 error generated.
make[2]: *** [src/libasr/CMakeFiles/asr.dir/codegen/asr_to_julia.cpp.o] Error 1
1 error generated.
make[2]: *** [src/libasr/CMakeFiles/asr.dir/codegen/asr_to_c.cpp.o] Error 1
1 error generated.
make[2]: *** [src/libasr/CMakeFiles/asr.dir/codegen/asr_to_cpp.cpp.o] Error 1
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/codegen/asr_to_wasm.cpp:10:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/asr_utils.h:12:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/utils.h:6:10: fatal error: 'filesystem' file not found
#include <filesystem>
         ^~~~~~~~~~~~
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/codegen/asr_to_x86.cpp:9:
In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/pass/do_loops.h:5:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_lang_lfortran/lfortran/work/lfortran-0.19.0/src/libasr/../libasr/utils.h:6:10: fatal error: 'filesystem' file not found
#include <filesystem>
         ^~~~~~~~~~~~
1 error generated.
make[2]: *** [src/libasr/CMakeFiles/asr.dir/codegen/asr_to_x86.cpp.o] Error 1
1 error generated.