build: Tell autoconf to use a macro dir

This avoids warnings coming from libtoolize and recent versions of
autoconf.

Makefile.am

@@ -20,6 +20,8 @@
 # FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 # IN THE SOFTWARE.
 
+ACLOCAL_AMFLAGS = -I m4
+
 SUBDIRS = include/epoxy src
 
 SUBDIRS += test

configure.ac

@@ -27,6 +27,7 @@ AC_INIT([libepoxy],
         [libepoxy])
 AC_CONFIG_SRCDIR([Makefile.am])
 AC_CONFIG_HEADERS([config.h])
+AC_CONFIG_MACRO_DIR([m4])
 
 # Initialize Automake
 AM_INIT_AUTOMAKE([foreign dist-bzip2 subdir-objects])