build-sys: Install bash completion to where bash-completion.pc says

build-sys: Install bash completion to where bash-completion.pc says

Fall back to the previous /etc/bash_completion.d dir on failures
(either old bash completion or not installed).

changes over Ville Skyttä's patch:
define PKG_CHECK_VAR macro which became available only in pkg-config 0.28

see https://bugs.freedesktop.org/show_bug.cgi?id=88782 and
https://bugs.freedesktop.org/show_bug.cgi?id=89540

Signed-off-by: Ville Skyttä <ville.skytta@iki.fi>
Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>