Bring Host build into separate project

build_projects/Microsoft.DotNet.Cli.Build.Framework/TargetConditions/EnvironmentAttribute.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Linq;
+
+namespace Microsoft.DotNet.Cli.Build.Framework
+{
+    [AttributeUsage(AttributeTargets.Method, AllowMultiple = true, Inherited = false)]
+    public class EnvironmentAttribute : TargetConditionAttribute
+    {
+        private string _envVar;
+        private string[] _expectedVals;
+
+        public EnvironmentAttribute(string envVar, params string[] expectedVals)
+        {
+            if (string.IsNullOrEmpty(envVar))
+            {
+                throw new ArgumentNullException(nameof(envVar));
+            }
+            if (expectedVals == null)
+            {
+                throw new ArgumentNullException(nameof(expectedVals));
+            }
+
+            _envVar = envVar;
+            _expectedVals = expectedVals;
+        }
+
+        public override bool EvaluateCondition()
+        {
+            var actualVal = Environment.GetEnvironmentVariable(_envVar);
+
+            if (_expectedVals.Any())
+            {
+                return _expectedVals.Any(ev => string.Equals(actualVal, ev, StringComparison.Ordinal));
+            }
+            else
+            {
+                return !string.IsNullOrEmpty(actualVal);
+            }
+        }
+    }
+}