build-sys: allow autoconf < 2.64 to be used

build-sys: allow autoconf < 2.64 to be used

Since commit 50d096a macro m4_ifblank is used, but as it is only
available in autoconf-2.64, on CentOS 6 system we end up with:

> $ ./autogen.sh
> configure:25396: error: possibly undefined macro: m4_ifblank
>       If this token and others are legitimate, please use
> m4_pattern_allow.
>       See the Autoconf documentation.
> [root@kir-ovz2 util-linux]# autoconf --version
> autoconf (GNU Autoconf) 2.63

So, the obvious thing to do would be to raise AC_PREREQ to 2.64
in configure.ac. But, given the facts that
 - autoconf 2.64 is not available for RHEL/CentOS 6,
 - the only need is one small macro,
it's better to just add the missing macro.

While at it, add the m4_ifnblank, too.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>