xcode warning with custom build steps

What are you trying to do?
We have post build steps setup to run custom install scripts (among other things).

What problem are you having?
Generally speaking, these work fine. But XCode outputs the following warning:

warning: Run script build phase 'Postbuild' will be run during every build because it does not specify any outputs.
To address this warning, either add output dependencies to the script phase, or configure it to run in every build
by unchecking "Based on dependency analysis" in the script phase.

Comparing xcode project files with and without that option set shows that the following (highlighted) line is added to the project file when it is set:

/* Begin PBXShellScriptBuildPhase section */
		9607AE3710C85E8F00CD1376 /* Postbuild */ = {
			isa = PBXShellScriptBuildPhase;
			alwaysOutOfDate = 1;                    << This is added/removed accordingly
			buildActionMask = 2147483647;
			files = (
			);
			inputPaths = (
			);
			name = Postbuild;
			outputPaths = (
			);
			runOnlyForDeploymentPostprocessing = 0;
			shellPath = /bin/sh;
			shellScript = "set -e\n blah blah blah \n";
		};
/* End PBXShellScriptBuildPhase section */

Is there anyway to control this from Premake?

What version of Premake are you using?
5.0.0-dev

Start by tracing Premake's Xcode project generation for PBXShellScriptBuildPhase and how custom post-build scripts are represented. Check whether the alwaysOutOfDate setting can be controlled from Premake; done means the generated Xcode project reflects that choice and removes the warning when appropriate.