Variables in smkfile

Variable substitution seems to be hard to avoid in most build script:
```
CFLAGS=-W -Wall -ansi -pedantic
gcc $(CFLAGS) -o hello.o -c hello.c 
gcc $(CFLAGS) -o main.o -c main.c 
gcc $(CFLAGS) -o hello hello.o main.o
```

Primary target is shell compatibility, but Makefile variable compatibility has to be considered at the same time.