Adds APPTAINER temporarily set to singularity to makefile

makefile

@@ -7,6 +7,8 @@ MAKEFLAGS += --warn-undefined-variables
 
 SIFS=$(addprefix sifs/,$(addsuffix .sif,$(notdir $(basename $(wildcard defs/*.def)))))
 
+APPTAINER=singularity
+
 BUILDDIR=sifs
 LOGDIR=logs
 
@@ -19,9 +21,6 @@ $(BUILDDIR):
 $(LOGDIR):
 	mkdir $(LOGDIR)
 
-apptainer:
-	ln -sfT `which apptainer` ./apptainer || ln -sfT `which singularity` ./apptainer
-
 $(BUILDDIR)/%.sif : defs/%.def $(BUILDDIR) $(LOGDIR) apptainer
-	./apptainer build --force $@ $< 2>&1 | tee $(addprefix $(LOGDIR)/,$(notdir $@)).`date +\%Y%m%d`.log
+	$(APPTAINER) build --force $@ $< 2>&1 | tee $(addprefix $(LOGDIR)/,$(notdir $@)).`date +\%Y%m%d`.log
 .PHONY: all