According to the official documentation for autoconf[1], the
correct names for these variables are abs_top_{src,build}dir
rather than abs_top{src,build}dir; in fact, we're already
using the correct names in various places, so let's just make
everything nice and consistent.

[1] https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Preset-Output-Variables.htmlSigned-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NMartin Kletzander <mkletzan@redhat.com>