Place Makefile 'configurables' up top

Variables that a package consumer might want to adjust should be placed
at the top of the file so they are immediately visible. Any constants
shall live below those (just the SRC folder, really).

Makefile

@@ -3,15 +3,15 @@
 ## Do not use it if that isn't your goal!
 ##
 
-SRC_DIR = ./src
-SRCS := $(wildcard $(SRC_DIR)/**)
-
 # Patch these to select a different build profile or target
 # The target shouldn't change any time soon. WASM64, I guess. Other targets
 # aren't aimed at the web, so you shouldn't be using this makefile.
 CARGO_TARGET := wasm32-unknown-unknown
 CARGO_PROFILE := tiny
 
+SRC_DIR = ./src
+SRCS := $(wildcard $(SRC_DIR)/**)
+
 .PHONY: clean full-clean web tarball
 
 web: out/asteroids.js out/asteroids_bg.wasm out/index.html