Using DwmEnableComposition

I have a Fortran application with an embedded OpenGL window. I need to call DwmEnableComposition to turn off Aero so that the buttons refresh correctly but I can't work out how to do it. I've added Dwmapi.lib to the linker line but still get

unresolved external symbol dwmiscompositionenabled referenced in function WinMain

My interface is

        integer(LRESULT) function DwmEnableComposition(c) bind(c)
            import
            integer(UINT), value :: c
        end function DwmEnableComposition

and my calling sequence is

         lret = DwmIsCompositionEnabled(compositionEnabled)
         if (compositionEnabled) then
             DWM_EC_DISABLECOMPOSITION = 0
             lret = DwmEnableComposition(DWM_EC_DISABLECOMPOSITION)
         else
             compositionEnabled = .false.
         end if

Any ideas greatly appreciated.