How to create deployed environment specific Ant build script

Question

I have a dbUser.properties file like below:

User_Dev=someuser1
Pass_Dev=pass1
User_Qa=someuser2
Pass_Qa=pass2
User_Prod=prodUser
Pass_Prod=pass3

Now I have to create a build file which will generate WAR file having dbUser.properties file as per environment, e.g. In Dev properties file will be like,

User=someuser1
Pass=pass1

For QA,

User=someuser2
Pass=pass2

For Prod,

User=prodUser
Pass=pass3

My question is how I can write Build.xml in ANT?