xdt-autogen: Properly handle whitespaces in filenames

and more generally special characters different from '\1'.

This is achieved by replacing the filename separator ' ' with '\1' in
lookup_configure_ac_*(), and the default IFS=' \t\n' with IFS='\1', but
only where necessary: the default IFS is usually required for things to
go as expected.

So we have to do an IFS round-trip substitution around each "for" loop
on filenames, just before and just after pathname expansion by the
shell.

Additionally, when running $XDT_PROG_ACLOCAL, one has to define some
new variables ACLOCAL_DIR and use conditional parameter expansion, to
properly set the program flags:
$XDT_PROG_ACLOCAL ${ACLOCAL_DIR:+-I "$ACLOCAL_DIR"}

Finally, one has to take care of some non optional quoting
(strictly speaking, some are optional in this commit, but I preferred
not to cut as close as possible and to keep an overall coherence).