build: Extend autobuild diagnostics by the username

* m4/autobuild.m4 (AB_INIT): Add username.
--

The old autobuild diagnostics show up in build logs.  What they are
missing is an information on the user who triggered a build.  EMAIL is
a common thing to denote the actual user using a service account.

m4/autobuild.m4

@@ -23,6 +23,9 @@ AC_DEFUN([AB_INIT],
 	if test "$hostname"; then
 	   AC_MSG_NOTICE([autobuild hostname... $hostname])
 	fi
+	if test "$EMAIL"; then
+	   AC_MSG_NOTICE([autobuild username... $EMAIL])
+	fi
 	ifelse([$1],[],,[AC_MSG_NOTICE([autobuild mode... $1])])
 	date=`date +%Y%m%d-%H%M%S`
 	if test "$?" != 0; then