[Shotwell] Build improvements

[Jens Georg <mail jensge org>]

Hi,

I just pushed some build changes that improve parallel build time
massively on my machine and non-parallel builds "a bit". 

Parallel make (-j5), before:

real    9m19.036s
user    30m18.868s
sys     1m55.768s

After:
real    3m43.677s
user    11m28.888s
sys     0m44.412s

Non-parallel make, before:

real    9m33.329s
user    8m49.024s
sys     0m38.344s

After:

real    8m10.280s
user    7m19.632s
sys     0m29.244s

I would appreciate if you can give it some thorough exercise