CustomLv2Host: fix debug mode

* Link pthread lib.
* Use absolute paths to read / write files on disk.
CustomLv2Plugin: add Lv2 Reader plugin
CustomLv2Host: throw runtime_error if an error occurred when create Node

* Throw runtime_error when can't find plugin.
* Throw runtime_error when can't instantiate plugin.
CustomLv2SearchPlugins: add option to show info of plugins which has no input

Useful when looking for generators.
CustomLv2Plugin: add 2 ports to reader plugin

* port 0 : atom port for the path of the input file (LV2_Atom_String)
* port 2 : control port for the number of channels of the audio input
* Update makefile to rebuild .so when update cpp
* Tmp : the path of the input file is written in the plugin... Need to
use the port 0 !
CustomLv2SearchPlugins: add filter to search plugins with atom port

The filter is : --atom