Bug#943752: Needs GCC-6

[Philipp Marek]

A further information I received via IRC is that memtest86+
_has_ to be compiled with GCC-6 - newer versions have a bug
that break memtest86+.

I have verified that GCC-6 gives a working result,
while GCC-8 doesn't.


So, basically,

    -CC?=gcc
    +CC:=gcc-6

in the Makefile.