@....@ how does it work ?

[Rod Butcher <rbutcher hyenainternet com>]

Can somebody explain or point me to some documentation on what's going 
on with lines like :-
CFLAGS = @CFLAGS@
in my Makefile.in (built via glade & then automake) which then gets 
translated into :-
CFLAGS = -g -O2
by configure.
So how/where from does @CFLAGS@ become -g -O2 (medium-optimised) ?
My test program works OK, but I want to know what's going on in the 
configure. I have the Goat Book text but can't find the @...@  bit. I 
presume it's a substitution, but from where ?
thanks
Rod
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