jni-gen, Add $compilerSuffix property

Very simple change that shouldn't interfere with the build process (please double check), but allows me to do:
```
ant -DcompilerSuffix=.exe -DndkSuffix=.cmd
// or (how I use it):
ant -f build-windows64.xml -DcompilerSuffix=.exe
ant -f build-android32.xml -DndkSuffix=.cmd
```
Without modifying the build files everytime.