include/prereq-build: fix ldconfig-stub symlink on macos and BSD OSs

include/prereq-build: fix ldconfig-stub symlink on macos and BSD OSs

Some OS may have the true bin to a different location than /bin/true.
BSD based system and macos have true on /usr/bin/true.

Fix this by checking both location and take the one available in the
system.

Reported-by: Sergey V. Lobanov <sergey@lobanov.in>
Suggested-by: Huangbin Zhan <zhanhb88@gmail.com>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>