If you include a file in your source with the {$I filename} directive, the compiler will look for it in the following places:
You can add files to the include file search path with the -I (see page 111) or -Fi (see page 110) options.
As an example, consider the following include statement in a file units/foo.pp:
{$i ../bar.inc}
Then the following command :
fpc -Iincfiles units/foo.pp
will cause the compiler to look in the following directories for bar.inc: