[1.2] make install does not install plugins

[wookietreiber]

The make install target does not install plugins. On a fresh install with 1.2 I get:

$ bcftools plugin -lv

No functional bcftools plugins were found. The environment variable BCFTOOLS_PLUGINS is not set.

Is there a default installation location for these plugins, where these are looked up?

I suggest installing them to $DESTDIR/$prefix/share/bcftools/plugins/*.so and automatically including this path as the default search path for plugins (PR pending).