UE4 hasx it’s own build system, rendering VS practically to text editor (UBT need VS compiler, but you can run UBT without VS openned). All UBT confiuration is done via xml file (i forget where it is :p) and more importently build scripts (C# files in module source directory, your project includes that).
I you want to change defines in *.Target.cs in your project, you add this to that function (if you dont have thatr function add it):
public override void SetupGlobalEnvironment(
TargetInfo Target,
ref LinkEnvironmentConfiguration OutLinkEnvironmentConfiguration,
ref CPPEnvironmentConfiguration OutCPPEnvironmentConfiguration
)
{
OutCPPEnvironmentConfiguration.Definitions.Add("WINVER=0x0603");
}