Explicitly state that used /bin/bash shell in Makefile

Code

    $(shell if [[ `tar --version` =~ "bsdtar" ]]; ...

written using Bash double-bracket conditions syntax.  When (default on
some platforms) /bin/sh interpreter used this code outputs error (but
luckily works as required):

    /bin/sh: 1: [[: not found
Use declaration-time expansion assignment for $(shell ...) variables

Not important in this case actually, but recommended for such cases in
general (so that shell expression evaluated only once).
Use make conditions instead bash conditions

Suggested by @jpommerening.
Add missed spaces around ':='
Merge branch 'fix_docs_makefile' of git://github.com/vrutsky/buildbot