Mac build instructions. These work on Catalina and Big Sur developer beta 5. They should be OK on Mojave at least as well.

Prereqs: install Xcode from the Mac App Store, and Homebrew following the instructions on https://brew.sh/

Open a Terminal window and install the necessary brew modules:
% brew install cmake
% brew install expat
% brew install zlib
% brew install qt

Get BletchMAME:
% git clone https://github.com/npwoods/bletchmame.git

This gets the BletchMAME source into a newly created folder called "bletchmame".

Currently (August 26, 2020) there's a minor edit you need to do to the source to make Clang happy. Use TextEdit or BBEdit or whatever to open the file lib/quazip/minizip_crypt.h and delete the word "register" on line 57, then save the file.

Setup for cmake:
% cd bletchmame
% mkdir build
% cd build

Run cmake (I recommend copy/pasting this command):
% cmake .. -DEXPAT_INCLUDE_DIR=/usr/local/opt/expat/include/ -DEXPAT_LIBRARY=/usr/local/opt/expat/lib/libexpat.a -DZLIB_ROOT=/usr/local/opt/zlib -DCMAKE_PREFIX_PATH=/usr/local/opt/qt5/

Once this completes, build:
% make -j5 (for a 4-core Mac; use -j3 for a 2-core or -j9 for an 8-core).

And run:
% ./BletchMAME

I haven't gotten BletchMAME to successfully launch MAME yet on the Mac, but it's interesting to play around with.