create a system default path

Make it possible to build qtchooser with a system default search path

build using make QTCHOOSER_GLOBAL_DIR=/usr/share/ will make qtchooser
fall back to search in /usr/share/qtchooser/ as a last option if
XDG_CONFIG_DIRS is not set

Change-Id: Ie0d438ddc1e259d4c5fb70aa332bd6bfef2448e5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>