configure.ac: set ACLOCAL_AMFLAGS to include build-aux

As suggested by libtoolize

Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Elie Tournier <elie.tournier@collabora.com>
Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>

Makefile.am

@@ -19,6 +19,8 @@
 # FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 # IN THE SOFTWARE.
 
+ACLOCAL_AMFLAGS = -I build-aux ${ACLOCAL_FLAGS}
+
 SUBDIRS = src tests
 
 if !OS_WIN32