If one file must be compiled with special options, you need to explicitly declare an obj target for that file and then use that target in your exe or lib target:

exe a : a.cpp b ;
obj b : b.cpp : <optimization>off ;

Of course you can use other properties, for example to specify specific compiler options:

exe a : a.cpp b ;
obj b : b.cpp : <cflags>-g ;

You can also use conditional properties for finer control:

exe a : a.cpp b ;
obj b : b.cpp : <variant>release:<optimization>off ;