Xcode does not support custom build commands

It seems the Xcode module does not handle custom build commands. We just switched to using them for 0 A.D.'s build system for the tests, and the Xcode test project doesn't work.

This is apparently the subject of a "forgotten" PR.

This can be tested by building 0 A.D., but the following "minimal" example works too:

workspace "Example"
	configurations { "Release" }
	platforms { "macOS" }
	location ("workspaces/")
	language "C++"
	targetdir ("build/output")
	objdir ("build/obj")
	nativewchar "Off"
	buildoptions { "-Wall", "-Wextra" }

project "Example"
	kind "ConsoleApp"

	files { "main.cpp" }

	filter "files:*.cpp"
		buildmessage 'Doing stuff'
		buildcommands {
			'touch somefile.cpp'
		}

		buildoutputs { 'somefile.cpp' }

Start by reviewing the forgotten premake-xcode PR #39 and reproducing the provided minimal workspace with Xcode. Compare its custom buildmessage, buildcommands, and buildoutputs behavior with the failing 0 A.D. test project; done means the generated Xcode project builds and runs the custom command correctly.