* c-common.c (c_common_post_options): Warn if -Wformat-zero-length
is used without -Wformat.
* c-common.h (warn_format_zero_length): Declare extern.
* c-decl.c (warn_options): Add "format-zero-length".
* c-format.c (warn_format_zero_length): Declare.
(set_Wformat): Set warn_format_zero_length for -Wformat.
(check_format_info): Only warn about zero-length formats if
warn_format_zero_length is true.  Include the format type
name in the warning message.
* doc/invoke.texi: Document -Wformat-zero-length.
* testsuite/gcc.dg/format/zero-length-1.c: New test.

From-SVN: r53592