`autoconf-2.72` tweaked `AC_CHECK_LIB` macro a bit and exposed missing
quoting in `configure.in` of `0verkill` as:
0verkill-unstable> ./configure: line 4182: syntax error near unexpected token `newline'
0verkill-unstable> ./configure: line 4182: `yes:'
`configure.in` requires a bit of patching. Let's pin it to older `2.69`
to allow `autoconf` upgrade to 2.72.