Makefile: save MAKE_VERSION

Save MAKE_VERSION as RUNNING_MAKE_VERSION since this is later clobbered
by the make package.

It will be used by the webkitgtk24 package to check for older make
versions which have a bug building it with parallel jobs (it hangs).

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>