I got it compiled on macOS with ``` meson _builddir -Dgtkdoc=false -Dcpp_std=c++17 && ninja -C _builddir ``` and Geany installs and runs fine.
The only problem I did run into during the compilation was inside `doc/meson.build` where the `cp` commands take the `-u` parameter which is not supported on macOS (for the compilation I just removed it).
One thing still missing is the support of the gtk-mac-integration library which is used to make GTK applications behave in a more native way. What's missing is the configuration option to enable mac integration and also the detection of the presence of the library like in the autotools m4 file here:
https://github.com/geany/geany/blob/master/m4/geany-mac-integration.m4
When the library is enabled, Geany should link against it and add `osx.h` and `osx.c` to the compilation:
https://github.com/geany/geany/blob/master/src/Makefile.am#L115