PSA: export XDG vars at build time

[Harald Sitter]

In case someone was not aware (because I was not :P), at build time
you need to have XDG_DATA_DIRS and XDG_CONFIG_DIRS exported if your
target prefix is not /usr.

Otherwise tools may not be able to find their data/config files. For
example meinproc5 looks up dtd files via QStandardPaths which entirely
relies on XDG on unix systems. Failing to export the XDG paths will
lead to very opaque build failures due to meinproc5.

HS