For compiling an application to a different widgetset on Linux I go to "Project Options" > "Additions and Overrides". Then, depending on whether I want to store the widgeset used in the project or session file (lpi vs lps) I click in line "Stored in project (.lpi)" or "Stored in session of project (.lps)", click "Set LCL WidgetType" and check the widgettypes that I want to support. Of course, the dependent libraries must have been set up for each widgetset.
This can be combined with Build Modes to make switching from WS to WS very easy.
The first compilation run after changing WS will take longer because all used units must be rebuilt for the new WS.