the build scriptlet doesn't take into account the source-subdir

I have a source in a subdir, and want to build and install it using scriptlets. Even if I declare the source-subdir tag, the scriptlets will run from the build dir. I think it would make more sense to run the scriptlets from the subdir instead.

Take a look at this part:

freeimage:
  source: http://golem.network/FreeImage.zip
  source-subdir: FreeImage
  plugin: nil
  build-packages: [g++]
  build: |
    cd FreeImage
    make -j$(SNAPCRAFT_PARALLEL_BUILD_COUNT)
  install: |
    mkdir -p $SNAPCRAFT_PART_INSTALL/usr/lib/
    cp FreeImage/Dist/* $SNAPCRAFT_PART_INSTALL/usr/lib/
    ln -s $SNAPCRAFT_PART_INSTALL/usr/lib/libfreeimage*.so $SNAPCRAFT_PART_INSTALL/usr/lib/libfreeimage.so

The subdir is FreeImage, and I still had to cd FreeImage during build, and during install I had to cp FreeImage/...

If we agree that the scriptlets should run from the subdir, a fix here is a little delicate because it will break the parts with subdir and scriptlets.