Eli Schwartz authored Dec 28, 2020 and Allan McRae committed Dec 29, 2020

In the autotools build, it only built in-tree, from cwd = doc/ and
resolving doc/../lib/libalpm

In the meson build, this accidentally worked if cwd =
pacman/builddir/ and resolved to builddir/../lib/libalpm/

But... this should always have been configured with the actual path to
the inputs. So, we will now proceed to do so.

Fixes building man3 if your out of tree builddir doesn't happen to be a
direct subdirectory of the source root.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Allan McRae <allan@archlinux.org>