Add cross-platform Start-PSBuild function

This can replace `build.ps1` and `build.sh`. However, we need to upload
packages so Travis can use PowerShell.

PowerShellGitHubDev.psm1

@@ -1,3 +1,57 @@
+function Start-PSBuild
+{
+    param(
+            [switch]$Restore,
+            [string]$Output = "$PSScriptRoot/bin"
+         )
+
+    if (-Not (Get-Command "dotnet" -ErrorAction SilentlyContinue)) {
+        throw "Build dependency 'dotnet' not found in PATH! See: https://dotnet.github.io/getting-started/"
+    }
+
+    $Top = "$PSScriptRoot/src/Microsoft.PowerShell.Linux.Host"
+    if ($Restore -Or -Not (Test-Path "$Top/project.lock.json")) {
+        dotnet restore
+    }
+
+    # TODO: use PowerShell to check OS when available
+    $Runtime = [System.Runtime.InteropServices.RuntimeInformation]
+    $OSPlatform = [System.Runtime.InteropServices.OSPlatform]
+    $Linux = $Runtime::IsOSPlatform($OSPlatform::Linux)
+    $OSX = $Runtime::IsOSPlatform($OSPlatform::OSX)
+    $Windows = $Runtime::IsOSPlatform($OSPlatform::Windows)
+
+    if ($Linux -Or $OSX) {
+        $InstallCommand = if ($Linux) { "apt-get" } elseif ($OSX) { "brew" }
+        foreach ($Dependency in "cmake", "g++") {
+            if (-Not (Get-Command $Dependency -ErrorAction SilentlyContinue)) {
+                throw "Build dependency '$Dependency' not found in PATH! Run '$InstallCommand install $Dependency'"
+            }
+        }
+
+        $Ext = if ($Linux) { "so" } elseif ($OSX) { "dylib" }
+        $Lib = "src/libpsl-native.$Ext"
+
+        Write-Host "Building $Lib"
+
+        pushd "src/libpsl-native"
+        cmake -DCMAKE_BUILD_TYPE=Debug .
+        make -j
+        ctest -V
+        if (-Not (Test-Path $Lib)) { throw "Compilation of $Lib failed" }
+        cp $Lib $Output
+        popd
+    }
+
+    Write-Host "Building PowerShell"
+
+    $Configuration =
+        if ($Linux -Or $OSX) { "Linux" }
+        elseif ($Windows) { "Debug" }
+
+    dotnet publish -o $Output -c $Configuration -f "netstandardapp1.5" $Top
+}
+
 function Start-DevPSGitHub
 {
     param(
@@ -18,14 +72,14 @@ function Start-DevPSGitHub
         $env:DEVPATH = $binDir
         if ($ZapDisable)
         {
-            $env:COMPLUS_ZapDisable = 1 
+            $env:COMPLUS_ZapDisable = 1
         }
 
-        if (!(Test-Path $binDir\powershell.exe.config))
+        if (-Not (Test-Path $binDir\powershell.exe.config))
         {
             $configContents = @"
-<?xml version="1.0" encoding="utf-8" ?> 
-<configuration> 
+<?xml version="1.0" encoding="utf-8" ?>
+<configuration>
     <runtime>
         <developmentMode developerInstallation="true"/>
     </runtime>
@@ -33,18 +87,18 @@ function Start-DevPSGitHub
 "@
             $configContents | Out-File -Encoding Ascii $binDir\powershell.exe.config
         }
-        
-        # splatting for the win      
+
+        # splatting for the win
         $startProcessArgs = @{
             FilePath = "$binDir\powershell.exe"
             ArgumentList = "$ArgumentList"
-        }  
+        }
 
         if ($NoNewWindow) {
             $startProcessArgs.NoNewWindow = $true
             $startProcessArgs.Wait = $true
         }
-        
+
         Start-Process @startProcessArgs
     }
     finally