[SV 65739] Add warning circular-dep.

[SV 65739] Add warning circular-dep.

Add a warning to control circular dependency detection.  Use "warn" as
the default action to be backward-compatible.

* src/warning.h (enum warning_type): Add warning type wt_circular_dep.
* src/warning.c (warn_init): Set default wt_circular_dep to w_warn.
* src/remake.c (update_file_1): Consult the circular-dep warning to
handle circular dependencies.
* tests/scripts/options/warn: Test --warn circular-dep flag.
* tests/scripts/variables/WARNINGS: Test .WARNINGS circular-dep flag.
* doc/make.texi: Document circular-dep warning.
* doc/make.1: Ditto.