Problem with .include

Whenever I try to use the include path to use for compiling scripts, ROOT seems to ignore it (or just not use it). Is there something special that needs to be given to the program other than just saying:

gROOT->ProcessLine(".include incdir");

The line:

gROOT->ProcessLine(".include")

Gives me all the correct directories I need, yet when I try to compile, none of the relative names of the header files seem to be found.

gSystem->AddIncludePath("-Iinclude_dir");Jan