How to get my iOS4.2 SDK developed code working with iOS3.1.3?

I am new to iPhone development. I started developing a project using iOS4.2 SDK. It works properly on both an iOS4.2 device and simulator. Later I realized that my app wasn't working on devices with iOS3.1.3. To solve this I updated the "Deployment Target" in my app's configuration file to "3.1.3".

The problem I am having is that I use UITapGestureRecognizer in some of my classes. On compiling my code using iOS 3.1.3 SDK I get the following errors:

error: 'UITapGestureRecognizer' undeclared (first use in this function)
error: 'tap' undeclared (first use in this function)

From what I have ready, this seems to be because iOS3.1.3 does not support UITapGestureRecognizer, so to run this i made the following changes wherever I use UITapGestureRecognizer:

Code:

 Class mailClass = (NSClassFromString(@"UITapGestureRecognizer"));
   if(mailClass != nil)
   {
     UITapGestureRecognizer *tap = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(findOutTheTagmap:)];
     [imageView addGestureRecognizer:tap];
     [tap release];
   }

I also updated my Target configuration and set the UIKit Framework to "Weak". Additionally I have also made the following changes:
Base SDK: 3.1.3
Compiler Version: GCC4.2 (also tried with GCC4.0)
Mac OS X Deployment Target: Compiler Default (also tried with 10.4 and 10.6)
iOS Deployment Target: 3.1.3

Yet, when I try compiling my code, I am still shown the same errors. What is surprising is that a few days ago someone was helping me and this worked.

I have tried pretty much everything I have found in forums.

Does anyone have an idea on how I can proceed?

Thanks in advance!