tools/gnulib: ignore AC_PREREQ error if actual version passes

tools/gnulib: ignore AC_PREREQ error if actual version passes

The gnulib-tool script is written to have a fatal error
whenever the minimum required version of autoconf
for the project that gnulib is being imported into
as defined in configure.ac was less than
the minimum required version required by gnulib.

However, none of this matters if the version of autoconf
that we use is newer than both requirements.

Instead, use functions from the bootstrap script
to check for the version of autoconf being used
and print a warning whenever this case occurs.

Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/15690
Signed-off-by: Robert Marko <robimarko@gmail.com>