A replacement for the NuGet.targets file that requires nuget.exe be on the path, so you don't have to commit it.

NuGet.targets.xml

<?xml version="1.0" encoding="utf-8"?>

<!-- ####    NOTE    #### -->
<!-- To build with Mono's xbuild on bash, you must execute [`sudo install-nuget.sh`](https://gist.github.com/2595337) first. -->

<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    <PropertyGroup>
        <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">$(MSBuildProjectDirectory)\..\</SolutionDir>

        <!-- Windows specific commands -->
        <PackagesConfig Condition=" '$(OS)' == 'Windows_NT'">$([System.IO.Path]::Combine($(ProjectDir), "packages.config"))</PackagesConfig>
        <PackagesDir    Condition=" '$(OS)' == 'Windows_NT'">$([System.IO.Path]::Combine($(SolutionDir), "packages"))</PackagesDir>
        <LocatorCommand Condition=" '$(OS)' == 'Windows_NT'">where</LocatorCommand>

        <!-- We need to launch nuget.exe with the mono command if we're not on windows -->
        <PackagesConfig Condition=" '$(OS)' != 'Windows_NT' ">packages.config</PackagesConfig>
        <PackagesDir    Condition=" '$(OS)' != 'Windows_NT'">$(SolutionDir)packages</PackagesDir>
        <LocatorCommand Condition=" '$(OS)' != 'Windows_NT'">which</LocatorCommand>
        
        <!-- NuGet command -->
        <NuGetCommand>nuget</NuGetCommand>

        <PackageOutputDir Condition="$(PackageOutputDir) == ''">$(TargetDir.Trim('\\'))</PackageOutputDir>

        <!-- Package sources used to restore packages. By default will used the registered sources under %APPDATA%\NuGet\NuGet.Config -->
        <PackageSources>""</PackageSources>

        <!-- Enable the restore command to run before builds -->
        <RestorePackages Condition="$(RestorePackages) == ''">false</RestorePackages>

        <!-- Commands -->
        <RestoreCommand>$(NuGetCommand) install "$(PackagesConfig)" -source $(PackageSources) -o "$(PackagesDir)"</RestoreCommand>

        <!-- Make the build depend on restore packages -->
        <BuildDependsOn Condition="$(RestorePackages) == 'true'">
            RestorePackages;
            $(BuildDependsOn);
        </BuildDependsOn>
    </PropertyGroup>
    
    <Target Name="CheckPrerequisites">
        <!-- Try to locate `nuget`. -->
        <Exec Command="$(LocatorCommand) $(NuGetCommand)" IgnoreExitCode="True">
            <Output TaskParameter="ExitCode" PropertyName="NuGetExeExitCode"/>
        </Exec>
        <!-- Raise an error if `nuget` wasn't located. -->
        <Error Text="Failed to locate `nuget`; please make sure your PATH environment variable includes a path to `nuget`." Condition="'$(NuGetExeExitCode)' != '' and '$(NuGetExeExitCode)' != '0'" />
    </Target>

    <Target Name="RestorePackages" DependsOnTargets="CheckPrerequisites">
        <Exec Command="$(RestoreCommand)"
              Condition="'$(OS)' != 'Windows_NT' And Exists('$(PackagesConfig)')" />
              
        <Exec Command="$(RestoreCommand)"
              LogStandardErrorAsError="true"
              Condition="'$(OS)' == 'Windows_NT' And Exists('$(PackagesConfig)')" />
    </Target>
</Project>