[AUCTeX] Installation Auctex & tools tuning

[pigreco erre]

Dear all,
I have a problem with my Auctex &  Emacs in LateX Preview functions.
I create & own portable installation and all programs are located in my USB together TexLive packages, Sumatra pdf-reader & all Ghostscript tools.
Takeing into account the item above I have two question to end my process:

    1) Where can I include Ghostscript path to allow the Latex preview functions go on in correct way? (I mean what    variable must include the Ghostscrip bin path and others binary file used in windows i.e. Ghostxps & Ghostpcl).

    2) What is the binary "rungs.exe" included in TeXlive that is called by preview-gs-command variable? (If I see the variable I find "f: (USB letter number)/Program-folders/TeXlive2015/bin/win32/rungs.exe").

I thank you to all in advance for your precious assistance
With my best regards
Alessandro