https://bugs.gentoo.org/563352
https://sourceforge.net/p/autogen/bugs/172/

this is fixed upstream in commit f303a1d5d8d8e002974eb64818f74041325f2433

--- a/pkg/libopts/mklibsrc.sh
+++ b/pkg/libopts/mklibsrc.sh
@@ -18,8 +18,7 @@
 ##  You should have received a copy of the GNU General Public License along
 ##  with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-set -ex
-exec 2> /tmp/mklibsrc-log.tx
+set -e
 
 top_builddir=`cd $top_builddir ; pwd`
 top_srcdir=`cd $top_srcdir ; pwd`