Modify "Conditional compilation symbols" from NuGet install.ps1 script

install.ps1

param($installPath, $toolsPath, $package, $project)

if ($project -ne $null) 
{
    $configuration = $project.ConfigurationManager | Where-Object {$_.ConfigurationName -eq "Debug"}

    if ($configuration -ne $null)
    {
        $oldValue = $configuration.Properties.Item("DefineConstants").Value

        $newValue = $newValue + ";" + "HELLO_WORLD"

        $configuration.Properties.Item("DefineConstants").Value = $newValue
    }

    $project.Save()
}

# Notes:
# - This example is way to simplistic
#
# Required modifications: 
# - check if the Symbol that you want to inject is already there.
# - check $project.IsDirty if needed

test.ps1

# In Package Manager Console:

$dte = [System.Runtime.InteropServices.Marshal]::GetActiveObject("VisualStudio.DTE.12.0")
$p = $dte.Solution.Projects | Where-Object {$_.ProjectName -eq "YourProjectName"}
# OR
$p = Get-Project

# And then...
$c = $p.ConfigurationManager | where-object {$_.ConfigurationName -eq "Release"}
$oldValue = $c.Properties.Item("DefineConstants").Value
$newValue = $oldValue + ";HELLO_WORLD"
$c.Properties.Item("DefineConstants").Value = $newValue
$p.Save()