Build fixes tcl commands (#817)

Build fixes tcl commands (#817)

* Link to gthread-2.0 when g_thread_init needs to be invoked

See https://developer.gnome.org/glib/stable/glib-Deprecated-Thread-APIs.html#g-thread-init:

> To use g_thread_init() in your program, you have to link
> with the libraries that the command `pkg-config --libs
> gthread-2.0` outputs. This is not the case for all the other
> thread-related functions of GLib. Those can be used without having to
> link with the thread libraries.

* Support building in a separate directory

* bindings/tcl/Makefile.am: use a relative path to the shlib in pkgIndex.tcl

* Add rrd_info support in Tcl bindings

* Add rrd_first support in Tcl bindings

* Fix memory leak in Rrd_Lastupdate