Update default compilation to C23 (from C11)

Configuration.mk

@@ -163,7 +163,13 @@ endif
 # the dos/microsoft lineage chose `.cpp` to address this same issue, leading to
 # confusion nowadays about the meaning of 'cpp'.]
 override ASFLAGS += -mthumb
-override CFLAGS  += -std=gnu11
+# '-gnu2x' is a deprecated alias for '-gnu23'. We're close enough in time still
+# (spring 2025) to the formal ratification of C23 (October 2024) that some
+# folks likely still have toolchains which only support the 2x _name_ for the
+# now-official C23 standard (even if the toolchain supports all the features
+# we care about under the 2x name). Eventually we should replace this with
+# explicit C23 selection.
+override CFLAGS  += -std=gnu2x
 override CPPFLAGS += \
       -frecord-gcc-switches\
       -gdwarf-2\

examples/tests/crash_dummy/main.c

@@ -2,12 +2,12 @@
 
 #include <libtock/interface/button.h>
 
-volatile int* nullptr = 0;
+volatile int* zeroptr = 0;
 
 static void button_callback(__attribute__ ((unused)) returncode_t ret,
                             __attribute__ ((unused)) int          btn_num,
                             __attribute__ ((unused)) bool         val) {
-  __attribute__ ((unused)) volatile int k = *nullptr;
+  __attribute__ ((unused)) volatile int k = *zeroptr;
 }
 
 int main(void) {

libtock-sync/display/text_screen.c

@@ -27,8 +27,7 @@ static void text_screen_size_cb(returncode_t ret, uint32_t width, uint32_t heigh
   result_size.height = height;
 }
 
-
-static returncode_t text_screen_op(returncode_t (*op)()) {
+static returncode_t text_screen_op(returncode_t (*op)(void (*)(returncode_t))) {
   returncode_t ret;
   result.fired = false;