Clarify and debug macro processor arithmetic in define statements.

Using

@# define a=2
@# define b=3
@# define p= a/b
disp(@{p})

with savemacro=temp onlymacro results in

@#line "junk2.mod" 1
disp(0)

which is not the expected outcome.

@# define a=1
@# define b=2/3
@# define p= a/b
disp(@{p})

crashes the preprocessor on Windows