Attempt to fix nightly builds

The fix introduced in dac5867af5ff90d4beb70fc30a5743f60f159e3a did not
work because autotools in our OE are too old. Use alternative way to
include custom m4 macros to fix it.

Change-Id: I5fe6d1180c2624cfe1d3673314f6846527a43464
diff --git a/openbsc/Makefile.am b/openbsc/Makefile.am
index f7eda56..31a54ea 100644
--- a/openbsc/Makefile.am
+++ b/openbsc/Makefile.am
@@ -1,5 +1,8 @@
 AUTOMAKE_OPTIONS = foreign dist-bzip2 1.6
 
+## FIXME: automake >= 1.13 or autoconf >= 2.70 provide better suited AC_CONFIG_MACRO_DIRS for configure.ac
+## remove line below when OE toolchain is updated to version which include those
+ACLOCAL_AMFLAGS = -I m4
 AM_CPPFLAGS = \
 	$(all_includes) \
 	-I$(top_srcdir)/include \