This is mostly a cosmetic issue, but it has led to an emacs bug report:

   https://debbugs.gnu.org/cgi/bugreport.cgi?bug=35629

The issue is that "gpg2 --version" has an extra "-unknown" in the output:

$ gpg2 --version
gpg (GnuPG) 2.2.15-unknown
[...]

I looked at the source, and it turns out that this is a consequence of running 
autoreconf.  You can fix it by building without autoreconf or, alternatively, by 
applying the following patch:

--- origsrc/gnupg-2.2.15/autogen.sh     2018-06-12 07:25:39.000000000 -0400
+++ src/gnupg-2.2.15/autogen.sh 2019-05-10 13:51:48.027196200 -0400
@@ -246,7 +246,7 @@ if [ "$myhost" = "find-version" ]; then
      else
        ingit=no
        beta=yes
-      tmp="-unknown"
+      tmp=""
        rev="0000000"
        rvd="0"
      fi

Ken
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