How to change compiler settings for one particular file ?

Select the
Sources line in the Build phase list of the concerned target (in the Targets tab), find out the desired source file name in the list, select it and then select in the Project menu the Show info item (or type command + I). Enter the required compiler flags in the now displayed window.

XCode (1.0.1 - native target):
Select the source file in the
Sources build phase of the desired target, then Get Info, select the Build tab of the displayed window and enter your additional flags.