Declare Makefile suffixes, to be a little closer to POSIX make compliance.

POSIX requires that suffixes used in implicit rules be declared.  Automake
allows one to do that by adding the suffixes to a variable named SUFFIXES.
We weren't doing that consistently, and this commit fixes that up.

POSIX doesn't allow suffixes to contain more than one period, so this isn't
a complete fix, since the Makefiles still use .h.pl and .inc.pl as
suffixes.  There are other reasons, too, why the PSPP Makefiles are not
POSIX-compliant.