Configuring: with-sysroot without-headers

[Matwey V. Kornilov]

Hi,

I am trying to compile cross (gcc version: 4.8.2, target: avr), and 
there is gap in my education. What is the real purpose of --with-sysroot 
and --with-build-sysroot?

When I use ./configure without --with-build-sysroot --with-sysroot 
--without-headers then everything looks to be compiled fine.

When I use ./configure --with-sysroot=/usr/avr 
--with-build-sysroot=/usr/avr --without-headers (which should do the 
same for my opinion), I got an error with the message "The directory 
that should contain system headers does not exist". Why does 
--without-headers not act as I expect? I will populate 
/usr/avr/usr/include with avrlibc later (should I?), but now I want not 
to use target headers (I have not got yet the cross to compile avr-libc).