build: fix most 'INCLUDES' is the old name for 'AM_CPPFLAGS'

not for bindings/Makefile.am, missing CXXFLAGS?
[warningectomy] unused variable (shadowed)

dia_xml.c:273:17: warning: unused variable 'ret' [-Wunused-variable]
      xmlDocPtr ret = xmlDoParseFile(fname, &error_xml);
                ^
dia_xml.c:300:15: warning: unused variable 'err' [-Wunused-variable]
  xmlErrorPtr err;
              ^
The first one was actually a bu which would have failed with files
missing encoding and not utf-8.
[warningectomy] expression result unused
[warningectomy] comparison of unsigned enum expression is always

arrows.c:2050:19: warning: comparison of unsigned enum expression < 0 is always false [-Wtautological-compare]
  if (arrow->type < 0 || arrow->type > MAX_ARROW_TYPE) {
      ~~~~~~~~~~~ ^ ~
arrows.c:2189:12: warning: comparison of unsigned enum expression >= 0 is always true [-Wtautological-compare]
  if (type >= 0 && type < MAX_ARROW_TYPE) {
      ~~~~ ^  ~
object.c:1116:25: warning: comparison of unsigned enum expression >= 0 is always true [-Wtautological-compare]
        dia_assert_true((h->id >= 0 && h->id <= HANDLE_MOVE_ENDPOINT)
                         ~~~~~ ^  ~
object.c:1121:26: warning: comparison of unsigned enum expression >= 0 is always true [-Wtautological-compare]
        dia_assert_true(h->type >= 0 && h->type <= NUM_HANDLE_TYPES,
                        ~~~~~~~ ^  ~
object.c:1126:34: warning: comparison of unsigned enum expression >= 0 is always true [-Wtautological-compare]
        dia_assert_true(h->connect_type >= 0
                        ~~~~~~~~~~~~~~~ ^  ~
[warningectomy] variable 'ret' is used uninitialized whenever ...

load_save.c:362:9: warning: variable 'ret' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
    if (strcmp((char *)val, "true")==0)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
load_save.c:368:10: note: uninitialized use occurs here
  return ret;
         ^~~
load_save.c:362:5: note: remove the 'if' if its condition is always true
    if (strcmp((char *)val, "true")==0)
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
load_save.c:358:15: note: initialize the variable 'ret' to silence this warning
  gboolean ret;
              ^
               = 0
[warningectomy] unused variable

disp_callbacks.c:176:14: warning: unused variable 'obj' [-Wunused-variable]
  DiaObject *obj;
             ^
display.c:142:14: warning: unused variable 'im_menu_item' [-Wunused-variable]
  GtkAction* im_menu_item;
             ^