buildworld not working with MAKEOBJDIRPREFIX

Hello,

I'm trying to build world as a regular user, using sources fetched into my home
directory and a different object directory. The rune I'm using to build is:

$ cd /home/royger/buildjob/freebsd
$ make -j30 buildworld MAKEOBJDIRPREFIX=/home/royger/buildjob/obj/

And this leads to the following build error:

--- all_subdir_rescue ---
--- cat.lo ---
cc -target x86_64-unknown-freebsd12.0 --sysroot=/home/royger/buildjob/obj//usr/home/royger/buildjob/freebsd/tmp -B/home/royger/buildjob/obj//usr/home/royger/buildjob/freebsd/tmp/usr/bin -O2 -pipe   -std=gnu99    -Qunused-arguments   -nostdlib -Wl,-dc -r -o cat.lo cat_stub.o /home/royger/buildjob/obj//usr/home/royger/buildjob/freebsd/rescue/rescue//usr/home/royger/buildjob/freebsd/bin/cat/cat.o
cc: error: no such file or directory: '/home/royger/buildjob/obj//usr/home/royger/buildjob/freebsd/rescue/rescue//usr/home/royger/buildjob/freebsd/bin/cat/cat.o'
*** [cat.lo] Error code 1

AFAIK this should work fine, does anyone has any clues about what causes this
failure?

Thanks, Roger.