Jakub,
this version makes the following changes to the earlier version.

*) Renames things to FOO_ver, rather than FOO_2

*) No attempt to deal with cross-version plugins and libgomp.

*) Adds GOMP_OFFLOAD_version function to plugin. (I went with your approach). 
Returns the GOMP_VERSION used to build the plugin, which libgomp checks matches 
the value for its build.  When we make incompatible changes to the plugin 
interface, that value can be incremented.

*) While working on gomp_load_plugin_for_device, I noticed the DLSYM  and 
DLSYM_OPT macros were somewhat funky.  We're loading functions, so don't expect 
a NULL value.  We can simply check the returned value and only need dlerror when 
we get NULL.  The counting that DLSYM_OPT does was somewhat funky too.  IMHO 
better for that macro to simply return a truth value.

ok for trunk?

nathan