As @Daniel Kutik mentioned, presetdef is a good option. Especially if one is working on a project with many build.xml files which one cannot, or prefers not to, edit (e.g., those from third-parties.)
To use presetdef, add these lines in your top-level build.xml file:
<presetdef name="javac">
<javac includeantruntime="false" />
</presetdef>
Now all subsequent javac tasks will essentially inherit includeantruntime="false". If your projects do actually need ant runtime libraries, you can either add them explicitly to your build files OR set includeantruntime="true". The latter will also get rid of warnings.
Subsequent javac tasks can still explicitly change this if desired, for example:
<javac destdir="out" includeantruntime="true">
<src path="foo.java" />
<src path="bar.java" />
</javac>
I'd recommend against using ANT_OPTS. It works, but it defeats the purpose of the warning. The warning tells one that one's build might behave differently on another system. Using ANT_OPTS makes this even more likely because now every system needs to use ANT_OPTS in the same way. Also, ANT_OPTS will apply globally, suppressing warnings willy-nilly in all your projects