Install relative symlinks instead of absolute ones.

This is useful when installing into a staging directory to avoid getting links
temporarily pointing out of that directory.

--- Makefile.am.orig	2020-02-09 14:53:55 UTC
+++ Makefile.am
@@ -168,9 +168,10 @@ TESTS = tests/test1 \
 		tests/unit_tests
 
 install-exec-hook :
-	ln -sf $(bindir)/par2$(EXEEXT) $(DESTDIR)$(bindir)/par2create$(EXEEXT)
-	ln -sf $(bindir)/par2$(EXEEXT) $(DESTDIR)$(bindir)/par2verify$(EXEEXT)
-	ln -sf $(bindir)/par2$(EXEEXT) $(DESTDIR)$(bindir)/par2repair$(EXEEXT)
+	cd $(DESTDIR)$(bindir)/ && \
+	ln -sf par2$(EXEEXT) par2create$(EXEEXT) && \
+	ln -sf par2$(EXEEXT) par2verify$(EXEEXT) && \
+	ln -sf par2$(EXEEXT) par2repair$(EXEEXT)
 
 uninstall-hook :
 	rm -f $(DESTDIR)$(bindir)/par2create$(EXEEXT)