msi: Add automatic codesigning
* src/Makefile.am (stamps/stamp-additional-signedfiles): Optionally additional signed files like gpgol.dll gpgex.dll * src/gpg4win.mk.in (AUTHENTICODE_sign): New (msi-signed): New target.
Codesigning can be configured in a sign.mk file with the
AUTHENTICODE variables for certificate / host etc.