Reference assemblies for .NET Framework v4.5 were not found
Setting up a build server on a new install of Windows Server 2012 R2 and ran into the following warning when building the projects:
Warning MSB3644 - The reference assemblies for framework ".NETFramework,Version=v4.5" were not found. To resolve this, install the SDK or Targeting Pack for this framework version or retarget your application to a version of the framework for which you have the SDK or Targeting Pack installed. Note that assemblies will be resolved from the Global Assembly Cache (GAC) and will be used in place of reference assemblies. Therefore your assembly may not be correctly targeted for the framework you intend.
The build server was a fresh machine and the issue was Visual Studio 2013 needing to be installed on the server. Installing Visual Studio Professional 2013 without any options resolved the issue.