I have cloned the git repository just yesterday, followed the instructions (adapting options.mk to linux) and could, without trouble, compile all files involved in the makefile within the home folder itensor.
Yet, the sample and tutorial makefiles throw errors. Although options.mk is loaded, there seems to be no mentioning of $(BLAS_ LAPACK_ LIBFLASG).
Adding this to the corresponding line, i.e.
$(APP): $(OBJECTS) $(ITENSOR_LIBS)
$(CCCOM) $(CCFLAGS) $(OBJECTS) -o $(APP) $(LIBFLAGS) $(BLAS_ LAPACK_ LIBFLASG)
did the job for me. I did not want to post this as a bug report, since I am still unsure whether I did something wrong.
(Please note that I added spaces after _ to avoid syntax interpretation)