Add -Wswitch to --enable-gcc-warnings

Add -Wswitch to --enable-gcc-warnings

Make --enable-gcc-warnings a bit pickier, by also using -Wswitch.
* configure.ac (WERROR_CFLAGS): Don’t add -Wno-switch.
* lib-src/etags.c (main, consider_token, C_entries):
* src/coding.c (encode_invocation_designation):
* src/data.c (Ftype_of):
* src/eval.c (Fdefvaralias, default_toplevel_binding)
(Fbacktrace__locals, mark_specpdl):
* src/lisp.h (record_xmalloc):
* src/syntax.c (scan_lists, scan_sexps_forward):
* src/window.c (window_relative_x_coord):
* src/xdisp.c (push_it, pop_it):
* src/xterm.c (xg_scroll_callback, x_check_fullscreen):
Error out or do nothing (as appropriate) if a switch statement
with an enum value does not cover all of the enum.
* src/dispextern.h (struct iterator_stack_entry.u.comp):
Remove unused member discovered by using -Wswitch.
* src/lisp.h (record_xmalloc): Add a ‘+ 0’ to pacify -Wswitch.
* src/vm-limit.c (check_memory_limits):
Simplify warning-diagnostic computation by using a table.