Add low-impact warning flags, fix warnings found by new flags

Add compiler flags: -Wundef, -Wfloat-equal, -Wredundant-decls, -Walloc-zero.

Fix -Wredundant-decls warnings:
- Remove stale cons_show_desktop_prefs declaration (ui.h)
- Remove duplicate connection_set_priority prototype (connection.h)
- Remove stale omemo_devicelist_configure_and_request declaration (omemo.h)

Fix potential buffer overflow in stanza.c:
- Increase pri_str and idle_str buffers from 10 to 12 bytes
  (INT_MIN = -2147483648 needs 12 bytes including NUL)

configure.ac

@@ -389,6 +389,8 @@ AM_CFLAGS="$AM_CFLAGS -Wall -Wextra -Wformat=2 -Wno-format-zero-length"
 AM_CFLAGS="$AM_CFLAGS -Wno-deprecated-declarations -Wno-unused-parameter -Wno-missing-field-initializers -Wno-sign-compare -Wno-cast-function-type"
 AM_CFLAGS="$AM_CFLAGS -Wnull-dereference -Wpointer-arith"
 AM_CFLAGS="$AM_CFLAGS -Wimplicit-function-declaration"
+AM_CFLAGS="$AM_CFLAGS -Wundef"
+AM_CFLAGS="$AM_CFLAGS -Wfloat-equal -Wredundant-decls"
 AM_CFLAGS="$AM_CFLAGS -fstack-protector-strong -fno-common"
 AM_CFLAGS="$AM_CFLAGS -D_FORTIFY_SOURCE=2"
 AM_CFLAGS="$AM_CFLAGS -std=gnu99 -ggdb3"
@@ -396,7 +398,7 @@ AM_CFLAGS="$AM_CFLAGS -std=gnu99 -ggdb3"
 # GCC-specific warnings (not supported by clang) — test each one
 saved_CFLAGS="$CFLAGS"
 for _flag in -Wlogical-op -Wduplicated-cond -Wduplicated-branches \
-             -Wstringop-overflow; do
+             -Wstringop-overflow -Warray-bounds=2 -Walloc-zero; do
     AC_MSG_CHECKING([whether $CC supports $_flag])
     CFLAGS="$saved_CFLAGS $_flag -Werror"
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM()],

src/ui/ui.h

@@ -322,7 +322,6 @@ void cons_wrap_setting(void);
 void cons_time_setting(void);
 void cons_wintitle_setting(void);
 void cons_notify_setting(void);
-void cons_show_desktop_prefs(void);
 void cons_states_setting(void);
 void cons_outtype_setting(void);
 void cons_intype_setting(void);

src/xmpp/connection.h

@@ -51,7 +51,6 @@ jabber_conn_status_t connection_register(const char* const altdomain, int port,
 void connection_set_disconnected(void);
 
 void connection_set_priority(const int priority);
-void connection_set_priority(int priority);
 void connection_set_disco_items(GSList* items);
 
 xmpp_conn_t* connection_get_conn(void);

src/xmpp/omemo.h

@@ -38,7 +38,6 @@
 #include "xmpp/iq.h"
 
 void omemo_devicelist_subscribe(void);
-void omemo_devicelist_configure_and_request(void);
 void omemo_devicelist_publish(GList* device_list);
 void omemo_devicelist_request(const char* const jid);
 void omemo_bundle_publish(gboolean first);

src/xmpp/stanza.c

@@ -1948,7 +1948,7 @@ stanza_attach_priority(xmpp_ctx_t* const ctx, xmpp_stanza_t* const presence, con
         return;
     }
 
-    char pri_str[10];
+    char pri_str[12];
     snprintf(pri_str, sizeof(pri_str), "%d", pri);
 
     xmpp_stanza_t* priority = xmpp_stanza_new(ctx);
@@ -2007,7 +2007,7 @@ stanza_attach_last_activity(xmpp_ctx_t* const ctx,
     xmpp_stanza_t* query = xmpp_stanza_new(ctx);
     xmpp_stanza_set_name(query, STANZA_NAME_QUERY);
     xmpp_stanza_set_ns(query, STANZA_NS_LASTACTIVITY);
-    char idle_str[10];
+    char idle_str[12];
     snprintf(idle_str, sizeof(idle_str), "%d", idle);
     xmpp_stanza_set_attribute(query, STANZA_ATTR_SECONDS, idle_str);
     xmpp_stanza_add_child(presence, query);