Stdlib build fails, due to dependency error or else confusion on where libsac lives

cd Stdlib/build
cmake ..
make -j

This dies with:

/home/sac/sac/BASE/Stdlib/src/system/src/RTClock/rtclock.c:11:39: fatal error: libsac/essentials/message.h: No such file or directory

Rerunning the make does not help.

ac@rattler:~/sac/BASE/Stdlib/build$ sac2c -V
sac2c 1.2-beta-BlackForest-611-g777dc
build-type: RELEASE
built-by: "sac" at 2018-01-01T15:26:55

I have just rebuilt sac2c_d and sac2c_r, and installed sac2c_r. Stdlib also cloned a few minutes ago.