Include the buildtype into the versioninfo.txt

* configure.ac (SHORTPRODUCT_NAME): New ac_subst.
* src/Makefile.am (versioninfo.txt): Use that instead of
  "buildsystem".

Having the buildsystem version (i.e. gpg4win version) in the
versioninfo.txt file helps to identify the installed version.
Because we have no downloaded package with a checksum we use zeroes
for the checksum and prefix the name with an underscore to indicate
that this is not a real package.