Replace 'echo' with the 'file' function from make in Makefile,

since 'echo' behaves differently on freebsd and debian with respect to backslashes.

This led to the following error while trying to compile the dtx file on freebsd:
(./automultiplechoice.sty
AMC version: 2018/05/02 v1.4.0\\string\nobreakspace {}beta1
! Undefined control sequence.
\\  ->\let \reserved@e
                        \relax \let \reserved@f \relax \@ifstar {\let
\reserv...
l.30 ...sPackage{automultiplechoice}[\AMC@VERSION]

!  ==> Fatal error occurred, no output PDF file produced!
Transcript written on automultiplechoice.log.