doc: update build instructions for Arch

`abs` has been deprecated, and shut down last month. [1]
`asp` replaces it, so rewrite the instructions to use this instead.

Also, add `--noextract` to the makepkg command, as there is no point
downloading and extracting the sources since they're not going to be
built here.

[1] https://www.archlinux.org/news/deprecation-of-abs/

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

doc/building.dox

@@ -148,10 +148,11 @@ $> sudo zypper modifyrepo --disable `zypper repos | grep source | awk '{print $5
 </li>
 <li><b>Arch</b>:
 <pre>
-$> abs extra/libinput
+$> sudo pacman -S asp
 $> cd $(mktemp -d)
-$> cp /var/abs/extra/libinput/PKGBUILD .
-$> makepkg --syncdeps --nobuild
+$> asp export libinput
+$> cd libinput
+$> makepkg --syncdeps --nobuild --noextract
 </pre>
 </li>
 </ul>