Add LV2 header location compatibility

Fix linking against libpthread

NULL terminate LV2Atom path - closes #7

Fix state:freePath Windows fallback

Declare supported/required features and options

Fix meta-data: document required LV2 features

Hide symbols by default

Compliant LV2 state path-mapping

lv2 state mandates that
>  The plugin MUST use this function [ absolute_path] in order to
>  actually open or otherwise use any paths loaded from plugin state.
Previously the plugin uses the value directly. Also
>  The caller is responsible for freeing the returned value with free().
is now implemented on systems other than windows (where this is not
possible, since the memory must be free()ed in the same module where it
was allocated.

Update reference to IR.LV2

Fix tiny mem-leak (free resampler)