Respect projects that say they only have one TFM (#1667)

Partially fixes dotnet/sdk#739.

The first iteration of cross-targeting support code unconditionally
queried each ProjectReference for the best TFM to build against, and
then explicitly specified that TFM when building the reference. This
caused a race condition when building a set of projects that had a
single-TFM project and another project that had a reference to it. The
entry point (probably .sln) build would build the referenced project
with no TF specified, and then the referencing project would build it
with an explicit TF specified. These two builds appeared different to
the MSBuild engine (because they had different sets of global
properties) and so were both executed, resulting in a race condition.

The fix is in two parts:
* Allow a project to report back through GetTargetFrameworkProperties
that it only has one TF to build.
* When a project reports that it has only one TF to build, issue its
build request without specifying any TF.

This commit is the latter.

src/XMakeTasks/Microsoft.Common.CurrentVersion.targets

@@ -1569,6 +1569,10 @@ Copyright (C) Microsoft Corporation. All rights reserved.
     <ItemGroup>
       <_MSBuildProjectReferenceExistent Condition="'%(_MSBuildProjectReferenceExistent.Identity)' == '%(Identity)' and '$(_ProjectReferenceTargetFrameworkProperties)' != ''">
         <SetTargetFramework>$(_ProjectReferenceTargetFrameworkProperties)</SetTargetFramework>
+
+        <UndefineProperties Condition="$(_ProjectReferenceTargetFrameworkProperties.Contains(`ProjectHasSingleTargetFramework=true`))">%(_MSBuildProjectReferenceExistent.UndefineProperties);TargetFramework;ProjectHasSingleTargetFramework</UndefineProperties>
+        <!-- Unconditionally remove the property that was set as a marker to indicate that for this call we should remove TargetFramework -->
+        <UndefineProperties Condition="!$(_ProjectReferenceTargetFrameworkProperties.Contains(`ProjectHasSingleTargetFramework=true`))">%(_MSBuildProjectReferenceExistent.UndefineProperties);ProjectHasSingleTargetFramework</UndefineProperties>
       </_MSBuildProjectReferenceExistent>
     </ItemGroup>