Loose GNU autoconf / automake requirements a bit

Now ngIRCd requires at least GNU autoconf 2.61 and automake 1.10, the
requirements of commit 67e882d4 have been too restrictive:

             19.2      67e882d4  now
  ---------- --------- --------- ---------
  autoconf   >=2.50    >=2.67    >=2.61
  automake   >=1.6     >=1.11    >=1.10

The tools required now are supported by Debian GNU/Linux 4.0 "Etch",
RedHat Enterprise Linux 5, and Mac OS X 10.6.x for example.

I read the changelogs of autoconf and automake, and I think ther's nothing
that prevents it from working with these older versions; and we don't want
to force users to upgrade without real benefits.

Please note: the recommended versions are still autoconf 1.11.x and the
most recent autoconf release that works with automake 1.11.x!