cairo: specify m4 location.

component/gtk-2/buildout.cfg

@@ -12,6 +12,7 @@ extends =
   ../libpng/buildout.cfg
   ../libtiff/buildout.cfg
   ../libtool/buildout.cfg
+  ../m4/buildout.cfg
   ../perl/buildout.cfg
   ../pkgconfig/buildout.cfg
   ../xorg/buildout.cfg
@@ -42,6 +43,7 @@ configure-options =
   --enable-fc
   --disable-lto
 environment =
+  M4=${m4:location}/bin/m4
   PATH=${freetype:location}/bin:${pkgconfig:location}/bin:${xz-utils:location}/bin:${autoconf:location}/bin:${automake:location}/bin:${libtool:location}/bin:%(PATH)s
   PKG_CONFIG_PATH=${:pkg_config_depends}
   CPPFLAGS=-I${zlib:location}/include