Port --enable-gcc-warnings to GCC 5.1 x86-64

Port --enable-gcc-warnings to GCC 5.1 x86-64

* lib-src/ebrowse.c (dump_sym):
* lib-src/hexl.c (main):
* src/ccl.c (ccl_driver):
* src/character.c (string_escape_byte8):
* src/dbusbind.c (xd_retrieve_arg, xd_add_watch):
* src/gnutls.c (Fgnutls_boot):
* src/gtkutil.c (xg_check_special_colors):
* src/image.c (x_build_heuristic_mask):
* src/print.c (safe_debug_print, print_object):
* src/term.c (produce_glyphless_glyph):
* src/xdisp.c (get_next_display_element)
(produce_glyphless_glyph):
* src/xterm.c (x_draw_glyphless_glyph_string_foreground):
Don't use a signed format to print an unsigned integer, or vice
versa.  GCC 5.1's new -Wformat-signedness option warns about this.
* src/image.c (png_load_body, jpeg_load_body):
Silence a bogus setjump diagnostic from GCC 5.1 (GCC bug 54561).