The compiler and other source-processing tools recognise the conditional compilation directives if/1, elif/1, else/0 and endif/0. The first two take a goal as their argument, and parts of the program source can be included or excluded depending of the satisfiability of that goal.
Note that these directives are the only ones that are not treated as predicate separators and can be placed around individual clauses of a predicate.
:- if(get_flag(hostarch,"i386_nt")).
...Windows-specific code...
:- elif( (get_flag(version_as_list,Version), Version @>= [6,0]) ).
...code for version 6.0 and later...
:- else.
...alternative code...
:- endif.