add back oneshell

Author: ethanndickson

Makefile

@@ -1,6 +1,7 @@
-# Use bash and immediately exit on failure
+# Use a single bash shell for each job, and immediately exit on failure
 SHELL := bash
 .SHELLFLAGS := -ceu
+.ONESHELL:
 
 # This doesn't work on directories.
 # See https://stackoverflow.com/questions/25752543/make-delete-on-error-for-directory-targets