`make buildworld` does not build base llvm on amd64

Hi,
I'm in the process of testing the drm-next-4.7 branch from the FreeBSDDesktop github repository on several computers. As I want to avoid building world on every machine, I decided to set up a dedicated build machine running FreeBSD 11.0-STABLE. The plan was to build world, kernel and some parts of the ports tree with poudriere, and then generate FreeBSD release images and a pkgng repository for binary packages.

I was able to build both the world and the kernel successfully. I was also able to generate working install images with the  make release command. However, it seems that the whole build process skips building the base versions of llvm39 and clang: as a consequence, all systems installed from the release images lack a compiler. Installing the new world and kernel on the build machine does not install llvm39: instead, llvm38 is left untouched, until flagged obsolete by the  make remove-old-libs command, and deleted.

Does anybody know why  make avoids building base llvm39 as a bootstrap compiler even while the current base version is llvm38 ? Is there a way to force the build of base llvm and clang ?

I do not use any src.conf or make.conf file. I read the src.conf(5) man page and tried to pass custom flags to  make (like  WITH_CLANG,  WITH_CLANG_BOOTSTRAP,  WITH_CLANG_FULL), without any success.

Here is how to reproduce:

1. Install FreeBSD 11.0-RELEASE or a snapshot of 12.0-CURRENT with default options. I tried the following with both versions and base llvm was not built in both case.

2. Install the devel/git package with  pkg.

3. Clone the github repository to /usr/src.

4. Try to build everything:
 # cd /usr/src/
# make buildworld buildkernel

5. The build stops immediately because  make cannot find llvm39. Here is the error message:
"You need to install devel/llvm39 to build or set XCC"

That error is new to me: I never saw it when I ran  make buildworld in previous releases. As a workaround, install devel/llvm39, or run  make with explicit environment variables:
 # make XCC=clang XCXX=clang++ XCPP=clang-cpp buildworld buildkernel

llvm39 will not be built whatever workaround you chose.

5. After the build finishes, make a release:
 # cd /usr/src/release
# make XCC=clang XCXX=clang++ XCPP=clang-cpp release WITH_DVD=1

6. Install the release to another computer, or install the new world and kernel on the current machine as you would normally do, with  mergemaster and  make remove-old /  make remove-old-libs.

7. Boot the new install or the new kernel, then run:
# cc -v
No compiler found.

Thanks a lot for your help!

Marin