[Solved] Build failing on MacOSX error: 'value' is unavailable: introduced in macOS 10.14

Its been a numbers of days now that building blender master fail for me.
building blender-v3.0-release branch works fine.

% sw_vers
ProductName:	Mac OS X
ProductVersion:	10.15.7
BuildVersion:	19H1615

terminal output:

% make

Configuring Blender in "/Users/dup/blender-source/build_darwin" ...
-- Detected OS X 11.1 and Command Line Tools 12.4 at /Library/Developer/CommandLineTools
-- SDKs Directory: /Library/Developer/CommandLineTools/SDKs
-- Detected OSX_SYSROOT: /Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk
-- JACK not found, disabling WITH_JACK
-- Using /Users/dup/blender-source/blender-git/../lib/darwin/openmp for OpenMP
-- SSE2 SIMD instructions enabled
-- Unsupported CXXFLAG: -Wc++20-designator
-- Unsupported CXXFLAG: -Wno-suggest-override
CUDA_TOOLKIT_ROOT_DIR not found or specified
-- CUDA compiler not found, disabling WITH_CYCLES_CUDA_BINARIES
-- Additionally falling back to dynamic CUDA load
-- Metal version too old, must be SDK 12.0 or newer, disabling WITH_CYCLES_DEVICE_METAL
-- Tests: Using Python executable: /Users/dup/blender-source/blender-git/../lib/darwin/python     /bin/python3.9
/Users/dup/blender-source/blender-git/source/blender/blenkernel/intern/idprop_serialize.cc:799:73: error: 'value' is
      unavailable: introduced in macOS 10.14
  const IDPropertySerializer &serializer = serializer_for(property_type.value());
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/optional:953:27: note: 'value' has been explicitly marked
      unavailable here
    constexpr value_type& value() &
[ 88%] Building C object source/blender/blenkernel/CMakeFiles/bf_blenkernel.dir/intern/idtype.c.o
[ 88%] Building C object source/blender/makesrna/intern/CMakeFiles/bf_rna.dir/rna_dynamicpaint_gen.c.o
1 error generated.
make[1]: *** [all] Error 2
make: *** [all] Error 2

Sorry but I fail to see what the task you linked has to do with this build failure.
The iMac were this is failing cannot be more up to date (cannot go more then macOS Catalina).

This should be fixed now:

thanks @brecht I can confirm it is fixed.