Fix build with missing .gnupg-autogen.rc
* src/Makefile.am (stamp-additional-signedfiles): Only sign if AUTHENTICODE_KEY is set. Read from gnupg-autogen.rc * src/gpg4win.mk.in (READ_AUTOGEN_template): Handle not existing ~/.gnupg-autogen.rc file.
This allows to either use "none" as AUTHENTICODE_KEY or
skip it if no gnupg-autogen.rc file exists.