Use UTF-8 encoding for process output on Mono

Fixes encoding issues when encoding property is not set: Refs fsprojects#1212.

This is a revised version of commit 2d63a45,
which broke external processes when standard error/output is not redirected: Refs fsprojects#1213

This commit only sets the encoding when output is redirected.

src/app/FakeLib/ProcessHelper.fs

@@ -7,6 +7,7 @@ open System.ComponentModel
 open System.Diagnostics
 open System.IO
 open System.Threading
+open System.Text
 open System.Collections.Generic
 open System.ServiceProcess
 
@@ -65,6 +66,9 @@ let ExecProcessWithLambdas configProcessStartInfoF (timeOut : TimeSpan) silent e
     if silent then 
         proc.StartInfo.RedirectStandardOutput <- true
         proc.StartInfo.RedirectStandardError <- true
+        if isMono then
+            proc.StartInfo.StandardOutputEncoding <- Encoding.UTF8
+            proc.StartInfo.StandardErrorEncoding  <- Encoding.UTF8
         proc.ErrorDataReceived.Add(fun d -> 
             if d.Data <> null then errorF d.Data)
         proc.OutputDataReceived.Add(fun d ->