Change winrt build to be less dependent on ANGLE

Now it does not try to build if the solution is not found. This way it's
possible to provide a minimal package with includes and libs and make it
build correctly.

Also remove messages from detect.py since it is ran for every platform
target.

platform/winrt/detect.py

@@ -19,11 +19,6 @@ def can_build():
         if (os.getenv("VSINSTALLDIR")):
 
             if (os.getenv("ANGLE_SRC_PATH") == None):
-                print("You need to define ANGLE_SRC_PATH to the path of ANGLE source root.")
-                return False
-
-            if not os.path.isfile(str(os.getenv("ANGLE_SRC_PATH")) + "/winrt/10/src/angle.sln"):
-                print ("Couldn't find the ANGLE solution. Is ANGLE_SRC_PATH configured to the right path?")
                 return False
 
             return True
@@ -58,6 +53,9 @@ def configure(env):
     jobs = str(env.GetOption("num_jobs"))
     angle_build_cmd = "msbuild.exe " + angle_root + "/winrt/10/src/angle.sln /nologo /v:m /m:" + jobs + " /p:Configuration=Release /p:Platform="
 
+    if os.path.isfile(str(os.getenv("ANGLE_SRC_PATH")) + "/winrt/10/src/angle.sln"):
+        env["build_angle"] = True
+
     if os.getenv('Platform') == "ARM":
 
         print "Compiled program architecture will be an ARM executable. (forcing bits=32)."