[warningectomy] format string is not a string literal

Sprinkle G_GNUC_PRINTF() to avoid the following warnings and get some new ones.

message.c:180:38: warning: format string is not a string literal [-Wformat-nonliteral]
  len = g_printf_string_upper_bound (fmt, args);
                                     ^~~
message.c:190:18: warning: format string is not a string literal [-Wformat-nonliteral]
  vsprintf (buf, fmt, args2);
                 ^~~
dialib.c:46:38: warning: format string is not a string literal [-Wformat-nonliteral]
  len = g_printf_string_upper_bound (fmt, args);
                                     ^~~
dialib.c:57:18: warning: format string is not a string literal [-Wformat-nonliteral]
  vsprintf (buf, fmt, args2);
                 ^~~
diacontext.c:162:27: warning: format string is not a string literal [-Wformat-nonliteral]
  msg = g_strdup_vprintf (format, args);
                          ^~~~~~
diacontext.c:181:27: warning: format string is not a string literal [-Wformat-nonliteral]
  msg = g_strdup_vprintf (format, args);
                          ^~~~~~
[warningectomy] format string is not a string literal (potentially insecure)

dia_xml.c:286:35: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
    dia_context_add_message (ctx, error_xml->message);
                                  ^~~~~~~~~~~~~~~~~~
diacairo-print.c:248:20: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
    message_error (error->message);
                   ^~~~~~~~~~~~~~
xfig-export.c:275:21: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
    message_warning(renderer->warnings[warning]);
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
load_save.c:1255:19: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
          message_error (error->message);
                         ^~~~~~~~~~~~~~
commands.c:507:35: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
    dia_context_add_message (ctx, error->message);
                                  ^~~~~~~~~~~~~~
[warningectomy] data argument not used by format string

load_save.c:1001:68: warning: data argument not used by format string [-Wformat-extra-args]
      dia_context_add_message (ctx, _("Error saving connections"), layer->name);
                                      ~~~~~~~~~~~~~~~~~~~~~~~~~~   ^
pixbuf.c:93:10: warning: data argument not used by format string [-Wformat-extra-args]
                              error->message);
                              ^
[warningectomy] format specifies type 'void *' but ...

pdf-import.cpp:328:7: warning: format specifies type 'void *' but the argument has type 'gint' (aka 'int') [-Wformat]
             GPOINTER_TO_INT (f), family, state->getTransformedFontSize(), scale);
             ^~~~~~~~~~~~~~~~~~~
                              ^
[warningectomy] format string is not a string literal (potentially insecure)

plug-ins.c:234:51: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
    message(_("%s Plugin could not be unloaded"), info->name);
                                                  ^~~~~~~~~~
[warningectomy] variable 'flip' is used uninitialized whenever ...

path-math.c:579:14: warning: variable 'flip' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
  } else if (distance_point_point (&eseg->p3,
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
path-math.c:590:7: note: uninitialized use occurs here
  if (flip) {
      ^~~~
path-math.c:579:10: note: remove the 'if' if its condition is always true
  } else if (distance_point_point (&eseg->p3,
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
path-math.c:569:16: note: initialize the variable 'flip' to silence this warning
  gboolean flip;
               ^
                = 0
[warningectomy] suggest braces around initialization of subobject

path-math.c:318:18: warning: suggest braces around initialization of subobject [-Wmissing-braces]
    Split sp = { 0 };
                 ^
                 {}
[warningectomy] no previous prototype for function

diapatternselector.c:74:1: warning: no previous prototype for function 'dia_pattern_selector_get_type' [-Wmissing-prototypes]
G_DEFINE_TYPE (DiaPatternSelector, dia_pattern_selector, GTK_TYPE_HBOX);
^
/usr/include/glib-2.0/gobject/gtype.h:1325:43: note: expanded from macro 'G_DEFINE_TYPE'
                                                            ^
/usr/include/glib-2.0/gobject/gtype.h:1465:60: note: expanded from macro 'G_DEFINE_TYPE_EXTENDED'
                                                            ^
/usr/include/glib-2.0/gobject/gtype.h:1682:7: note: expanded from macro '_G_DEFINE_TYPE_EXTENDED_BEGIN'
GType \
      ^
<scratch space>:81:1: note: expanded from here
dia_pattern_selector_get_type
^