build-sys: use $PKG_CONFIG instead of calling the pkg-config binary directly
build-sys: don't hard code bash-completion directory
build-sys: replace backticks `` with $()

for consistencies sake use $() everywhere
build-sys: resolve absolute path for the dbus directories

/usr/share/dbus-1/system-services simply looks a lot nicer then
/usr/share/dbus-1/services/../system-services