When compiling with G_LOG_USE_STRUCTURED, g_message(), g_debug(), etc.
use g_log_structured(). The message format string and its format
arguments are passed as the final set of arguments in a longer varargs
list, which includes the log domain and level (and other) fields.
Passing the message format in this way means it’s not possible for the
compiler to know to check its format placeholders when compiling with
-Wformat.

Fix support for this by adding a new semi-private helper function,
_g_log_structured_standard(), which only uses varargs for the message
format and its arguments, and uses fixed arguments for the other fields.
This is then converted to a set of GLogFields and passed to
g_log_structured() as normal.

Support for -Wformat when compiling *without* G_LOG_USE_STRUCTURED was
never broken.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=793074