configure.ac: make sure m4 macros are included in the build

This change prevents the following error to occur when calling
autoreconf:

  >>> scrub 2.6.1 Autoreconfiguring
  cd /work/build/scrub/build/scrub-2.6.1/ &&  PATH="/work/build/scrub/host/bin:/work/build/scrub/host/sbin:/work/build/scrub/host/usr/bin:/work/build/scrub/host/usr/sbin:/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/lib/jvm/default/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/sbin:/usr/sbin" AR="/usr/bin/ar" AS="/usr/bin/as" LD="/usr/bin/ld" NM="/usr/bin/nm" CC="/usr/bin/gcc" GCC="/usr/bin/gcc" CXX="/usr/bin/g++" CPP="/usr/bin/cpp" OBJCOPY="/usr/bin/objcopy" RANLIB="/usr/bin/ranlib" CPPFLAGS="-I/work/build/scrub/host/usr/include" CFLAGS="-O2 -I/work/build/scrub/host/usr/include" CXXFLAGS="-O2 -I/work/build/scrub/host/usr/include" LDFLAGS="-L/work/build/scrub/host/lib -L/work/build/scrub/host/usr/lib -Wl,-rpath,/work/build/scrub/host/usr/lib" PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 PKG_CONFIG="/work/build/scrub/host/usr/bin/pkg-config" PKG_CONFIG_SYSROOT_DIR="/" PKG_CONFIG_LIBDIR="/work/build/scrub/host/usr/lib/pkgconfig:/work/build/scrub/host/usr/share/pkgconfig" INTLTOOL_PERL=/usr/bin/perl ACLOCAL="/work/build/scrub/host/usr/bin/aclocal -I /work/build/scrub/host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/share/aclocal" AUTOCONF="/work/build/scrub/host/usr/bin/autoconf" AUTOHEADER="/work/build/scrub/host/usr/bin/autoheader" AUTOMAKE="/work/build/scrub/host/usr/bin/automake" AUTOPOINT=/bin/true /work/build/scrub/host/usr/bin/autoreconf -f -i -I "/work/build/scrub/host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/share/aclocal" -I "/work/build/scrub/host/usr/share/aclocal"
  libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, 'config'.
  libtoolize: copying file 'config/ltmain.sh'
  libtoolize: Consider adding 'AC_CONFIG_MACRO_DIRS([m4])' to configure.ac,
  libtoolize: and rerunning libtoolize and aclocal.
  libtoolize: Consider adding '-I m4' to ACLOCAL_AMFLAGS in Makefile.am.
  configure.ac:10: error possibly undefined macro: X_AC_META
        If this token and others are legitimate, please use m4_pattern_allow.
        See the Autoconf documentation.
  configure.ac:11: error possibly undefined macro: X_AC_EXPAND_INSTALL_DIRS
  configure.ac:51: error possibly undefined macro: AC_PKGCONFIG
  configure.ac:70: error possibly undefined macro: X_AC_CHECK_PTHREADS
  autoreconf: /work/build/scrub/host/usr/bin/autoconf failed with exit status: 1
  package/pkg-generic.mk:185: recipe for target '/work/build/scrub/build/scrub-2.6.1/.stamp_configured' failed
  make: *** [/work/build/scrub/build/scrub-2.6.1/.stamp_configured] Error 1
  make: Leaving directory '/work/repos/buildroot'

Signed-off-by: Samuel Martin <s.martin49@gmail.com>

configure.ac

@@ -7,6 +7,7 @@ AC_INIT(m4_esyscmd([awk '/Name:/ {printf "%s",$2; exit}' META]),
 AC_CONFIG_AUX_DIR([config])
 AC_CONFIG_SRCDIR([NEWS])
 AC_CANONICAL_SYSTEM
+AC_CONFIG_MACRO_DIR([config])
 X_AC_META
 X_AC_EXPAND_INSTALL_DIRS