Build Avogadro's plugins as static by default

Change-Id: I014be4ae4b1cf80de8b43a7492078ca43a74c682
Added AVOGADRO_PLUGIN_DIR as an env variable

Check AVOGADRO_PLUGIN_DIR, and override any other base directory if it
is set. This makes testing more reliable, and could be useful where the
relative pathing is not desirable.

Change-Id: I536c078fe7a0429be2e2cec4586c69c77c729b13
Merge topic 'plugin-paths' into master

ab1b8ec5 Added AVOGADRO_PLUGIN_DIR as an env variable
735012b4 Build Avogadro's plugins as static by default