avr linking with precompiled library

Hi,
I just tried the upcoming precompiled library feature. I love it, but I noticed a possible bug.

I created a sketch with my own 3 libraries included (HWLib HWLibDriver SiBase), but I kept getting undefined reference errors while linking.

While diving into the problem I noticed the reason:

Link command:
```
"C:\Program Files (x86)\Arduino\hardware\tools\avr/bin/avr-gcc" -w -Os -g -flto -fuse-linker-plugin -Wl,--gc-sections,--relax -mmcu=atmega2560 "-LC:\Program Files (x86)\Arduino\libraries\HWLib\src\atmega2560" **-lHWLib -lHWLibDriver -lSiBase** -o "C:\Users\corja\AppData\Local\Temp\arduino_build_576204/sketch_dec10a.ino.elf" "C:\Users\corja\AppData\Local\Temp\arduino_build_576204\sketch\sketch_dec10a.ino.cpp.o" "C:\Users\corja\AppData\Local\Temp\arduino_build_576204/..\arduino_cache_568285\core\core_arduino_avr_mega_cpu_atmega2560_0c812875ac70eb4a9b385d8fb077f54c.a" "-LC:\Users\corja\AppData\Local\Temp\arduino_build_576204" -lm
```

Error:
```
C:\Users\corja\AppData\Local\Temp\ccHHlJFj.ltrans0.ltrans.o: In function `setup':
C:\Users\corja\Desktop\sketch\sketch_dec10a/sketch_dec10a.ino:12: undefined reference to `si_hw_lib_tick'
collect2.exe: error: ld returned 1 exit status
```

Notice that my libraries are included before the object files of the sketch itself.
Since ordering is very important during linking, this is causing the problem

Changing it to this fixes it:
```
"C:\Program Files (x86)\Arduino\hardware\tools\avr/bin/avr-gcc" -w -Os -g -flto -fuse-linker-plugin -Wl,--gc-sections,--relax -mmcu=atmega2560 "-LC:\Program Files (x86)\Arduino\libraries\HWLib\src\atmega2560" -o "C:\Users\corja\AppData\Local\Temp\arduino_build_576204/sketch_dec10a.ino.elf" "C:\Users\corja\AppData\Local\Temp\arduino_build_576204\sketch\sketch_dec10a.ino.cpp.o" "C:\Users\corja\AppData\Local\Temp\arduino_build_576204/..\arduino_cache_568285\core\core_arduino_avr_mega_cpu_atmega2560_0c812875ac70eb4a9b385d8fb077f54c.a" "-LC:\Users\corja\AppData\Local\Temp\arduino_build_576204" **-lHWLib -lHWLibDriver -lSiBase** -lm
```
I'm sorry if I am repeating a known bug.

Corjan