missing: don't cater to 'configure.in' in our advice The use of 'configure.in' as autoconf input now deprecated the development versions of both Automake and Autoconf, and we'd like to speed up its demise. * missing: In the advice displayed in case some autotools are detected to be missing, refer to 'configure.ac' unconditionally, instead of referring to 'configure.in' when that file exists in the current directory (and no 'configure.ac' does). Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>