Parallel build is broken for STATIC_BUILD=ON

[rosik]

How to reproduce

$ export MAKEFLAGS=j
$ tarantoolctl rocks install kafka STATIC_BUILD=ON
...
make[2]: *** No rule to make target 'librdkafka/lib/librdkafka.a', needed by 'kafka/tntkafka.so'. Stop.
Makefile:129: recipe for target 'all' failed
make: *** [all] Error 2

How it affects others

In the other project (central) rocks install is called from the top-level cmake build. There we install multiple rocks so we want to do it in parallel (like cmake && make -j). Cmake sets the MAKEFLAGS environment variable thus building kafka fails.