fixed: Compile Source Code Occur Errors On Mac Machine

Frameworks/MQTTnet.NetStandard/MQTTnet.Netstandard.csproj

@@ -1,7 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard1.3;netstandard2.0;net452;net461;uap10.0</TargetFrameworks>
+    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">netstandard1.3;netstandard2.0;net452;net461;uap10.0</TargetFrameworks>
+    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard1.3;netstandard2.0</TargetFrameworks>
     <AssemblyName>MQTTnet</AssemblyName>
     <RootNamespace>MQTTnet</RootNamespace>
     <GeneratePackageOnBuild>False</GeneratePackageOnBuild>
@@ -14,9 +15,9 @@
     <Description />
     <Authors />
     <PackageId />
-    <SignAssembly>true</SignAssembly>
+    <!-- <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>codeSigningKey.pfx</AssemblyOriginatorKeyFile>
-    <DelaySign>false</DelaySign>
+    <DelaySign>false</DelaySign> -->
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(TargetFramework)' == 'uap10.0'">