I finally succeeded.
==>
First step was to update GCC to 4.8 from 4.7.
it seemed to give the built-in functions 32bits for ARM which were missing for atomic.h needed in mmo.h.
But I still had problems for 64bits functions. After one day, I thought "anyway Raspberry isn't 64bits" and simply "swiched off" the functions by returning a random floating value.
Compiler had problems, but exe wouldn't use them on raspberry. And that's it, it's working.
So 3 things can prevent you from compiling rathena on raspberry:
1 - ./configure doesn't work, got some errors with zlib even if it's installed. Rather go for CMake
2 - Make install doesn't work, nullpo.c gets errors on stupid variables, because depending on implementations of C, the variable is either a pointer, or a structure. Try casting it as a pointer. It worked for me (multiple modifications in file)
3 - mempool.h got some problems in atomic.h where you need to "switch off" 64bits functions (and gcc >=4.8 absolutely needed) by deleting call to built-in functions for int64 which doesn't work for raspberry.
That's all folks, hope it helps anyone trying the same thing.
Currently I was running on raspbian. Server's working but memory available is low. That's bad since i'm only using terminal, i mean on 512mo I only have 1xx free after loading system. Too much things on this system.
==> there's a fork by Darkbasic which only uses 7mo of ram after start. It doesn't come with X, Xorg, LXDE. No GUI. But fairenough. And less shared memory for graphics. I'll try this this week. I'm sure there's a good potential on that; it'd be good enough for my RO server =).
Bye !