Build issue on MacOS + workaround

[macbutch]

Building on MacOS yields:

...
[ 50%] Building CXX object CMakeFiles/gui.dir/src/Mahi/Gui/Native.cpp.o
/Users/mark/dev/mark/sfs/gui/build/_deps/mahi-gui-src/src/Mahi/Gui/Native.cpp:25:10: fatal error: 'filesystem' file not found
#include <filesystem>

I was able to figure out that this relates to the Native dialogs and also the <filesystem> header is not available in my environment. I have been able to work around this (for now) by disabling the native dialogs. Ideally CMake should be finding my <filesystem> headers (I think they're there); I also noticed that it's set to use C++14 but <filesystem> is a C++17 thing (I think).