The Makefiles on that repo have pretty weird hardcoded system include files. Looks like the original author had the haiku_source.hpkg mounted under /system/develop/sources? (not sure, in any case, not something one would expect).
Seems to me that decorators use quite a few private headers, so compiling them outside of the Haiku tree looks to be more troublesome than just have them integrated on-tree (as a branch of the main Haiku repo).
Nah, bravo ! 
DISCLAIMER :
it is for older or patient readers.
Just skip it if you hate long posts ! 
…………………………………………………….
I wondered it had started again a usual everlasting debate on
getting market ( for an open source project)
and
wider userbase {for a beta stage OS that despite of this :
Meanwhile appears a not quite chance - that was rided by (social) media as well - so dropped here to get users who would sail away from Win11.
There were many true statements - those partially even - about who would ship away , who would remain on Windows, who will get scared and/or switch to Linux due to professional reasons (if they can do it - however it may not mentioned above the reasons) and may try out Haiku (remain or fall back to any Linux distro/Mac/Windows).
Some will deal with dual boot, some remain on VM installs.
I think this is not important.
The huge help would be if somewhere , in some country a business would be started up (mirracle!) , that would create some HW or SW that would be useful for Haiku,
and part of the profit could be reinvested into Haiku developers or services support.
Until then that miracle happens ( :)) ) we should thank many for those who could and can continously donate developers directly or via Haiku Inc. offered possibilities.
Also , we users, - more widely - may should take time to learn and follow to try out what shared here or Haiku site to probe and resolve or at least report issues.
I write down an example for you.
I use USB keys to have installed Haiku. For daily driving I started to use a Beta4 32bit (I tried since beta3 but that cannot be used on the older laptops reliably so I gave op on then).
Althoug after a drive issue that corrupted my well configured install, I switched to 64bit, and after created a nightly of it on another same vendor/size thumbdrive to be able to use much more browser and better stability , new features that were rather on 64bit or explicitly in nightly.
I had to accomodate to EFI boot, so I created first 2 partitions on the USB thumbdrive ( a 256 GB size) and copied the whole content from EFI partition of installer USB onto freshly installed Haiku USB thumbdrive’s haiku_esp partition.
I never copied the new EFI loader to the EFI partition, as in the installation instructions.
This way I always used the provided one from the EFI partition in the installer image.
I migrated both R1B4 and such nightly to a same size Samsung usb thimbdrive, but 512 GB, now I created 4 MS-DOS primary partitions
1 EFI - fat32
1 boot - bfs
1 data, media - bfs (with bigger chunks than default)
1 shared data - fat32 (for anything that would copy between R1B4/B5 and nightly or other OS)
I was quite prepared, at least I thought so, as I also created an independent backup USB thumbdrive with BFS formatted, as I was used only Haiku for daily driving at that time already, so I could read/write it from Haiku only was no problem. Some important configs, docs, pics, others were saved there.
So, I created the migration with a simple Haiku install onto new, bigger thumbdrives’ boot partition, but before moved bigger data and media files to the saparate data partiton from the home directory, before the Haiku install, so the install finished well and reasonably faster. Also it had not failed due to the big files, as I experienced that in some cases earlier.
I was really satisfied with the results - hitting my shoulder for the good jobs -, and used the both Haiku happily.
Then came R1B5 … I upgraded both R1B4 and R1nightly with new repos - renamed the hostnames … and worked well !.. for a while …
Once I updated both Haiku on the storage keys in this year’s early springtime and I got a KDL about ”BFS panic !.. on CPU 4…” etc.
I cannot like to report KDLs as I have no smarthone to take a good picture about it, so i wanted to tinkering with, especially as i had previous keys with backup state, I could lost some good links and docs, but some could be recreated or find on backup drive.
OK, so, I thought
→ HW issue – in that case it was not (PC had not reported related errors when used diagnosing capabilities)
→ BFS corruption - I checked using the spare 256 GB keys : there were nothing error with checkfs, bfsinfo
For letting me calm I switched back to R1B4 that was on the 256 GB keys, but it had not let me give up without further investigation to resolve the issue, so I finally thought I take a try to replace EFI bootloader in the EFI partition it may allow me more features during boot time.
I saved the R1B4 EFI bootloader on the EFI partition and copied / renamed the correspinding available bootloader files in the /boot filesystem (using the instructions on Haiku site) both on the Haiku Beta stick and later the Haiku Nightly stick.
I mounted only one ESP at a time and one Haiku boot partition. Fortunately I use identical boot partition labels.
Right, finally I tested first the R1B5 stick, I wanted to see which new features may enable me to save logs or if KDL happens, saves my ass.
Surprisingly it was so few time to navigate in options, once just Haiku boot menu navigation interrupted and followed a boot, and bumm … it has started to boot … and I finally reached desktop !
Ok, so new EFI bootloader required to skip “BFS panic!..” KDL message.
The step I made to have a better investigation tool for me - finally solved the issue.
I admit - I was lucky, but I ment problem solving ability this way above..
Kind regards,