Location of system headers

[Soubhik Bhattacharya]

hi!
i've gcc 2.93.3 installed in my i586-mandrake-linux system. how do i know
what are the locations searched by gcc for system headers? also, is there
a way to change it (not for a particular invokation of gcc but
permanently, ie to affect all the subsequent invokations)? i've gone thru
the documentation. but have not found any hint. may be i've missed
something.......
your help will be highly appreciated...
TIA,
soubhik.