libraries/libvirt: detect bash-completion properly

and install the completion file to systemwide directory

libraries/libvirt/libvirt.SlackBuild

@@ -47,8 +47,8 @@ VIRTGROUP=${VIRTGROUP:-users}
 
 # Some optional dependencies checks
 audit="dis" ; if pkg-config --exists audit ; then audit="en" ; fi
-bash_completion="" ; if pkg-config --exists bash_completion ; then
-  bash_completion="-Dbash_completion=enabled -Dbash_completion_dir=/etc/bash_completion.d"
+bash_completion="" ; if pkg-config --exists bash-completion ; then
+  bash_completion="-Dbash_completion=enabled -Dbash_completion_dir=/usr/share/bash-completion/completions"
 fi
 libiscsi="dis" ; if pkg-config --exists libiscsi ; then libiscsi="en" ; fi