Makefile: Simplify variable defaults
Makefile: Simplify variable defaults

Use `?=` operator.

Also stop exporting `$CURDIR`, this isn't being used anywere as an env
variable.

jkozol/welder-web,weldr/welder-web,jkozol/welder-web,jkozol/welder-web,weldr/welder-web,weldr/welder-web,jkozol/welder-web,weldr/welder-web