Define a macro directory in the autotools build

As libtool suggest we do; this also allows using ACLOCAL_AMFLAGS inside
build that create a separate environment with their own m4 macros, like
jhbuild and Cerberus.

In order to avoid a warning from autoreconf, we create the empty macros
directory during the autogen.sh step; aclocal would create the directory
for us anyway, but different versions of autotools may have different
behaviours.

Closes: #58

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

autogen.sh

@@ -6,6 +6,8 @@ test -z "$srcdir" && srcdir=.
 ORIGDIR=`pwd`
 cd "$srcdir"
 
+mkdir m4 || exit 1
+
 autoreconf -v --install || exit $?
 cd "$ORIGDIR" || exit $?
 

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 -Wno-portability dist-xz no-dist-gzip tar-ustar subdir-objects])