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()],