Autoconf 2.70 or later is required to properly handle multi-line
AC_CHECK_FUNCS lists. The recent changes in ruby#13907 that split function
lists across multiple lines require autoconf 2.70's improved whitespace
handling.

Without autoconf 2.70+, the configure script generation fails with syntax
errors like:
../configure: line 29306: syntax error near unexpected token `pthread_attr_get_np'

This is because autoconf 2.69 and earlier don't properly handle multi-line
function lists in AC_CHECK_FUNCS without backslash continuations, while
2.70+ requires square brackets and handles newlines correctly.