This is works not only with the Mars release, but also previous releases, as
well.

Open Window/Preferences.
Find PDE/Target Platform
Select your (active) target platform
Click Edit
Click Add
Select "Software Site"
Click Next
In "Work With" type: http://download.eclipse.org/eclipse/updates/4.7 (replace 4.7 with your current version)
Check "Eclipse RCP Target Components"
Check "Equinox Target Components"
Uncheck "Include required software"
Check "Include all environments"
Press Finish
Press Finish
Press OK

Open your product file and select the “Export” option. You will see that the
“Export for multiple platforms” checkbox is available.