Fix build on Linux

.NET Core does not support building for `net45` target on non-Windows platforms. See dotnet/sdk#335
To workaround this need to set up environment variable `FrameworkPathOverride` with path to `/usr/lib/mono/4.5/`

Senod issue that portable profile was looked on wrong path so just disable it on Linux/Mac builds.

Author: xplicit

.vscode/tasks.json

@@ -5,10 +5,15 @@
     "command": "dotnet",
     "isShellCommand": true,
     "args": [],
+    "options": {
+        "env": {
+            "FrameworkPathOverride": "/usr/lib/mono/4.5/"
+        }
+    },    
     "tasks": [
         {
             "taskName": "build",
-            "args": [ "tests/ServiceStack.Text.Tests" ],
+            "args": [ "src/ServiceStack.Text.sln", "-v", "m" ],
             "isBuildCommand": true,
             "showOutput": "silent",
             "problemMatcher": "$msCompile"

src/ServiceStack.Text/ServiceStack.Text.csproj

@@ -1,7 +1,14 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net45;netstandard1.1;netstandard1.3;portable45-net45+win8</TargetFrameworks>
+    <TargetFrameworks>net45;netstandard1.1;netstandard1.3</TargetFrameworks>
+  </PropertyGroup>
+
+  <PropertyGroup Condition=" '$(OS)' != 'Unix' ">
+    <TargetFrameworks>$(TargetFrameworks);portable45-net45+win8</TargetFrameworks>
+  </PropertyGroup>
+
+  <PropertyGroup>
     <!--<TargetFrameworks>net45;netstandard1.1;netstandard1.3;portable45-net45+win8;sl5</TargetFrameworks>-->
     <AssemblyName>ServiceStack.Text</AssemblyName>
     <PackageId>ServiceStack.Text</PackageId>