Makefile: try to be robust against shell meta-chars in filenames.

Makefile: try to be robust against shell meta-chars in filenames.

* Makefile.am: In each rule body, unless a make variable needs
to be seen as multiple words by the shell (for word-splitting
loops, or because it contains several commands), quote it. Use
single quotes unless the shell needs to perform variable inter-
polation, in which case use double quotes.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>