configure.ac: fix autoreconf with autoconf >= 2.70

Fix the following build failure raised with openocd and autoconf >= 2.70
due to
http://git.savannah.gnu.org/gitweb/?p=autoconf.git;a=commit;h=aba75f6d4a9c875a9d5d90a07c6b3678db66a4bf:

>>> openocd 0.11.0 Autoreconfiguring
autoreconf: error: configure.ac: AC_INIT not found; not an autoconf script?

Fixes:
 - http://autobuild.buildroot.org/results/5fb7aa28703aff61ba850eac11bd35c8804528ae



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