[SV 40657] Reinstate old behavior for suffix rules with prereqs

[SV 40657] Reinstate old behavior for suffix rules with prereqs

POSIX says that suffix rules cannot have prerequisites, but after
making this change we observed a number of makefiles "in the wild"
that were relying on this behavior and failed.

For .POSIX: makefiles, obey POSIX.  Otherwise preserve the old
behavior.  However, generate a warning so users know this is a
problem.  In a future version we will change all behavior to be
POSIX-conforming.

* NEWS: describe the change
* src/rule.c (convert_to_pattern): If posix_pedantic don't make a
pattern rule if prereqs exist.  Otherwise show a warning.
* tests/scripts/features/suffixrules: Add tests for the new behavior
including .POSIX vs. non-.POSIX.