Testsuite: compatibility fix for older MATLAB

addpath does not deal well with relative path on older MATLABs.

Makefile

@@ -8,4 +8,4 @@ check-octave:
 	$(OCTAVE) --no-init-file --silent --no-history --eval "addpath([pwd() '/../dseries/src']); addpath([pwd() '/../src']); runtest;" && [ -f my/report/dir/report.pdf ]
 
 check-matlab:
-	@$(MATLAB) -nosplash -nodisplay -r "addpath dseries/src; addpath src; cd test; runtest; quit" && [ -f test/my/report/dir/report.pdf ]
+	@$(MATLAB) -nosplash -nodisplay -r "addpath([pwd '/dseries/src']); addpath([pwd '/src']); cd test; runtest; quit" && [ -f test/my/report/dir/report.pdf ]