kmod fails to build in xenial (and yakkety)

[Matthias Klose]

kmod fails to build in xenial (and yakkety), using ppa:ubuntu-toolchain-r/ppa

seen at
http://people.ubuntuwire.org/~wgrant/rebuild-ftbfs-test/test-rebuild-20160614-xenial.html

 debian/rules build
gtkdocize --docdir libkmod/docs
autoreconf --force --install --symlink
libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, 'build-aux'.
libtoolize: linking file 'build-aux/ltmain.sh'
libtoolize: putting macros in AC_CONFIG_MACRO_DIRS, 'm4'.
libtoolize: linking file 'm4/libtool.m4'
libtoolize: linking file 'm4/ltoptions.m4'
libtoolize: linking file 'm4/ltsugar.m4'
libtoolize: linking file 'm4/ltversion.m4'
libtoolize: linking file 'm4/lt~obsolete.m4'
configure.ac:13: installing 'build-aux/compile'
configure.ac:19: installing 'build-aux/config.guess'
configure.ac:19: installing 'build-aux/config.sub'
configure.ac:17: installing 'build-aux/install-sh'
configure.ac:17: installing 'build-aux/missing'
Makefile.am: installing 'build-aux/depcomp'
Makefile.am:207: installing 'build-aux/py-compile'
parallel-tests: installing 'build-aux/test-driver'
libkmod/docs/gtk-doc.make:33: error: EXTRA_DIST must be set with '=' before using '+='
libkmod/docs/Makefile.am:29: 'libkmod/docs/gtk-doc.make' included from here
autoreconf: automake failed with exit status: 1
debian/rules:60: recipe for target 'configure' failed
make: *** [configure] Error 1
dpkg-buildpackage: error: debian/rules build gave error exit status 2