Sounds hackish but that's the only way I found to force it to generate the
functions in the .cxx files with fluid 1.3...

Index: ZynAddSubFX-2.4.1/src/UI/BankUI.fl
===================================================================
--- ZynAddSubFX-2.4.1.orig/src/UI/BankUI.fl
+++ ZynAddSubFX-2.4.1/src/UI/BankUI.fl
@@ -32,7 +32,7 @@ decl {\#include "../Misc/Config.h"} {pub
 
 class BankProcess_ {} {
   Function {process()} {open return_type {virtual void}
-  } {}
+  } {code{;}}
   decl {Bank *bank;} {public
   }
 } 
Index: ZynAddSubFX-2.4.1/src/UI/OscilGenUI.fl
===================================================================
--- ZynAddSubFX-2.4.1.orig/src/UI/OscilGenUI.fl
+++ ZynAddSubFX-2.4.1/src/UI/OscilGenUI.fl
@@ -129,7 +129,7 @@ delete [] spc;} {selected
 
 class PSlider {: {public Fl_Slider}
 } {
-  Function {PSlider(int x,int y, int w, int h, const char *label=0):Fl_Slider(x,y,w,h,label)} {} {}
+  Function {PSlider(int x,int y, int w, int h, const char *label=0):Fl_Slider(x,y,w,h,label)} {} {code{;}}
   Function {handle(int event)} {return_type int
   } {
     code {int X=x(),Y=y(),W=w(),H=h();
Index: ZynAddSubFX-2.4.1/src/UI/PartUI.fl
===================================================================
--- ZynAddSubFX-2.4.1.orig/src/UI/PartUI.fl
+++ ZynAddSubFX-2.4.1/src/UI/PartUI.fl
@@ -85,7 +85,7 @@ end();} {}
 
 class PartUI_ {} {
   Function {showparameters(int kititem,int engine)} {return_type virtual
-  } {}
+  } {code{;}}
 } 
 
 class PartKitItem {: {public Fl_Group}
Index: ZynAddSubFX-2.4.1/src/UI/PresetsUI.fl
===================================================================
--- ZynAddSubFX-2.4.1.orig/src/UI/PresetsUI.fl
+++ ZynAddSubFX-2.4.1/src/UI/PresetsUI.fl
@@ -18,9 +18,9 @@ decl {\#include "../Params/Presets.h"} {
 
 class PresetsUI_ {} {
   Function {refresh()} {open return_type {virtual void}
-  } {}
+  } {code{;}}
   Function {~PresetsUI_()} {open return_type virtual
-  } {}
+  } {code{;}}
 } 
 
 class PresetsUI {} {