Use simply-defined make variables.

test/val/Makefile

@@ -2,11 +2,11 @@
 # makefile for the regression tests that return an error code on failure
 
 ifneq ($(shell echo),)
-  CMD_EXE = 1
+  CMD_EXE := 1
 endif
 
-CC65FLAGS = -t sim6502
-SIM65FLAGS = -x 200000000
+CC65FLAGS := -t sim6502
+SIM65FLAGS := -x 200000000
 
 CL65 := $(if $(wildcard ../../bin/cl65*),../../bin/cl65,cl65)
 SIM65 := $(if $(wildcard ../../bin/sim65*),../../bin/sim65,sim65)
@@ -17,7 +17,7 @@ else
 RM := rm -f
 endif
 
-WORKDIR := ./../../testwrk
+WORKDIR := ../../testwrk
 
 .PHONY: all clean