[Makefilea] Fix for GNU make < 4

[Makefilea] Fix for GNU make < 4

Closes #9387

The shell assignment operator != was introduced in GNU make 4.0, or
specifically the commit in [1]. This fix removes such usages and
fallback to a more portable syntax. Tested with:

* GNU make 3.82 on CentOS 7.2
* bmake 20150910 on CentOS 7.2, source RPM from Fedora 24 [2]
* GNU make 4.2.1 on Arch Linux (Arch official package)
* bmake 20160926 on Arch Linux (Arch official package)
* GNU make 3.82 on Arch Linux (Compiled from source)
* Apple bsdmake-24 on macOS Sierra, binary package from Homebrew

Thanks @bdeyal for the feedback of the first tests

[1] http://git.savannah.gnu.org/cgit/make.git/commit/?id=b34438bee83ee906a23b881f257e684a0993b9b1
[2] http://koji.fedoraproject.org/koji/buildinfo?buildID=716769