In config/defaults.mk, use '=' to set {OCCA,RAJA}_DIR.
In config/defaults.mk, use '=' to set {OCCA,RAJA}_DIR.

This makes it easier to configure mfem by copying defaults.mk to
user.mk and editting it: if using '?=', the value given in user.mk
will not overwrite the one from defaults.mk.

mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem