configure.ac: fix autoreconf with autoconf >= 2.70

Drop AC_CONFIG_AUX_DIR to avoid the following build failure with autoconf >= 2.70:

autoreconf: error: cannot create config/autoconf: No such file or directory

Fixes:

• http://autobuild.buildroot.org/results/433599039d6e1ee301465f9867e169c121a0646f

Signed-off-by: Fabrice Fontaine fontaine.fabrice@gmail.com