Add cflags

Think about reading from input such as "make.O2.g.Wall.pedantic.Wextra":
After compilation of the Makefile we could have another program to read
and write to the new Makefile. It would use cmd-line args and parse for
CFLAGS. If none are provided (i.e. just "make") then we would remove
that empty CFLAGS = line. Else CFLAGS = -O2 -g -Wall -pedantic -Wextra

I can't think of a way to do this all in one step. Unless we have
test2.c use cmd-line args such as "gcc test2.c" "./a.out O2 -g -Wall
-pedantic -Wextra" and then append these to the generated Makefile.