MTR does not use /dev/shm with a out-of-source build

I have a separate disk where I store all git and bzr trees: /media/DATA/maria-bzr/.

There is a convenience symlink from home directory:

cd ~

ln -s /media/DATA/maria-bzr/  .

Now I clone a tree:

cd maria-bzr

bzr clone bzr+ssh://bazaar.launchpad.net/~maria-captains/maria/10.0/

And compile an out-of-source build:

cd 10.0

mkdir bld

cd bld

cmake .. -DCMAKE_BUILD_TYPE=Debug

make -j8

Now I run a test:

cd mysql-test

./mtr --mem ctype_utf8

It ignores the --mem option and creates a real var subdirectory under the current directory, instead of symlinking it to a random sub directory at /dev/shm/.
As a result, the entire ./mtr runs about 4 hours instead of 20 minutes.

Note, it works as expected when I use a in-source tree:

cd ~/maria-bzr/10.0/

rm -rf bld

cmake . -DCMAKE_BUILD_TYPE=Debug

make -j8

cd mysql-test

./mtr  --mem ctype_utf8