Remove .la files generated by autotools

[Robert Ancell]

If you build a part that depends on a library part, e.g.

parts:
glib:
plugin: autotools
source: https://download.gnome.org/core/3.20/3.20.2/sources/glib-2.48.1.tar.xz
glib-networking:
plugin: autotools
source: https://download.gnome.org/core/3.20/3.20.2/sources/glib-networking-2.48.2.tar.xz
after:
- glib

Then the dependant part fails to link due to the .la files being moved around:

CCLD glib-pacrunner
libtool: warning: library '/home/bob/bzr/gnome-3-20-snap/stage/lib/libgio-2.0.la' was moved.
libtool: error: cannot find the library '/lib/libgobject-2.0.la' or unhandled argument '/lib/libgobject-2.0.la'

.la files aren't commonly used (they are removed in Debian) and removing them fixes this issue. See discussion http://stackoverflow.com/questions/7104494/building-so-module-with-autotools-libtool-without-la-and-a-variants-being-ins

[Robert Ancell (robert-ancell)]

https://github.com/snapcore/snapcraft/pull/670