Verify MSBuild version in Developer CMD prompt

Roslyn is designed to have the simplest possible contribution story:
clone then build. Every pre-req needed is either located on the machine
or bootstrapped via NuGet. All the way down to using an xcopy MSBuild if
needed.

The one case which causes a problem is the VS command prompt. In this
case MSBuild is pre-installed on the machine and may or may not be
suitable for building Roslyn.

Previously when building from a VS command prompt we just used whatever
MSBuild was provided. The assumption being a developer command prompt
was an explicit statement of whath MSBuild you wanted to use. Based on
all of our customer reports though this does not seem to be the
assumption that consumers of our repo have. The build gave them no
explicit errors about the provided toolset and hence when the build
failed they assigned flakiness to our repo.

Going forward we are applying the same version validation to MSBuild
when provided via a developer command prompt. If it doesn't match we
will refuse to build asking the user to upgrade VS or build from a
normal command prompt.

build/scripts/build-utils.ps1

@@ -312,15 +312,21 @@ function Get-MSBuildKindAndDir([switch]$xcopy = $false) {
         return
     }
 
-    # MSBuild from an active VS command prompt.  
+    # MSBuild from an active VS command prompt. Use the MSBuild here so long as it's from a 
+    # compatible Visual Studio. If not though throw and error out. Given the number of 
+    # environment variable changes in a developer command prompt it's hard to make guarantees
+    # about subbing in a new MSBuild instance
     if (${env:VSINSTALLDIR} -ne $null) {
         $command = (Get-Command msbuild -ErrorAction SilentlyContinue)
-        if ($command -ne $null) {
+        if ((Test-SupportedVisualStudioVersion ${env:VSCMD_VER}) -and ($command -ne $null) ) {
             $p = Split-Path -parent $command.Path
             Write-Output "vscmd"
             Write-Output $p
             return
         }
+        else {
+            throw "Developer Command Prompt for VS $(${env:VSCMD_VER}) is not recent enough. Please upgrade or build from a normal CMD window"
+        }
     }
 
     # Look for a valid VS installation
@@ -353,6 +359,19 @@ function Get-MSBuildDir([switch]$xcopy = $false) {
     return $both[1]
 }
 
+
+# Dose this version of Visual Studio meet our minimum requirements for building.
+function Test-SupportedVisualStudioVersion([string]$version) { 
+    if (-not ($version -match "^([\d.]+)(\+|-)?.*$")) { 
+        Write-Host "here"
+        return $false
+    }
+
+    $V = New-Object System.Version $matches[1]
+    $min = New-Object System.Version "15.3"
+    return $v -ge $min;
+}
+
 # Get the directory and instance ID of the first Visual Studio version which 
 # meets our minimal requirements for the Roslyn repo.
 function Get-VisualStudioDirAndId() {
@@ -365,10 +384,8 @@ function Get-VisualStudioDirAndId() {
         # set of SDK fixes. Parsing the installationName is the only place where this is 
         # recorded in that form.
         $name = $obj.installationName
-        if ($name -match "VisualStudio(Preview)?/([\d.]+)(\+|-).*") { 
-            $minVersion = New-Object System.Version "15.3.0"
-            $version = New-Object System.Version $matches[2]
-            if ($version -ge $minVersion) {
+        if ($name -match "VisualStudio(Preview)?/(.*)") { 
+            if (Test-SupportedVisualStudioVersion $matches[2]) {
                 Write-Output $obj.installationPath
                 Write-Output $obj.instanceId
                 return