Use _Countof to implement lenof, where available.

Up-to-date trunk clang has introduced a built-in operator called
_Countof, which is like the 'lenof' macro in this code (returns the
number of elements in a statically-declared array object) but with the
safety advantage that it provokes a compile error if you accidentally
use it on a pointer. In this commit I add a cmake-time check for it,
and conditional on that, switch over the definition of lenof.

This should add a safety check for accidental uses of lenof(pointer).
When I tested it with new clang, this whole code base compiled cleanly
with the new setting, so there aren't currently any such accidents.

clang cites C2y as the source for _Countof: WG14 document N3369
initially proposed it under a different name, and then there was a big
internet survey about naming (in which of course I voted for lenof!),
and document N3469 summarises the results, which show that the name
_Countof and/or countof won. Links:

  https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3369.pdf
  https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3469.htm

My reading of N3469 seems to say that there will _either_ be _Countof
by itself, _or_ lowercase 'countof' as a new keyword, but they don't
say which. They say they _don't_ intend to do the same equivocation we
had with _Complex and _Bool, where you have a _Countof keyword and an
optional header file defining a lowercase non-underscore macro
wrapping it. But there hasn't been a new whole draft published since
N3469 yet, so I don't know what will end up in it when there is.

However, as of now, _Countof exists in at least one compiler, and that
seems like enough reason to implement it here. If it becomes 'countof'
in the real standard, then we can always change over later. (And in
that case it would probably make sense to rename the macro throughout
the code base to align with what will become the new standard usage.)

charset/internal.h

@@ -6,7 +6,13 @@
 #define charset_internal_h
 
 /* This invariably comes in handy */
-#define lenof(x) ( sizeof((x)) / sizeof(*(x)) )
+#ifndef lenof
+#if HAVE_COUNTOF
+#define lenof(x) _Countof(x)
+#else
+#define lenof(x) ( (sizeof((x))) / (sizeof(*(x))))
+#endif
+#endif
 
 /* This is an invalid Unicode value used to indicate an error. */
 #define ERROR 0xFFFFL                  /* Unicode value representing error */

cmake/cmake.h.in

@@ -5,6 +5,8 @@
 
 #cmakedefine NO_MULTIMON
 
+#cmakedefine01 HAVE_COUNTOF
+
 #cmakedefine01 HAVE_WINRESRC_H
 #cmakedefine01 HAVE_WINRES_H
 #cmakedefine01 HAVE_WIN_H

cmake/setup.cmake

@@ -116,6 +116,12 @@ int main(int argc, char **argv) {
     free(p);
 }" HAVE_ALIGNED_ALLOC)
 
+check_c_source_compiles("
+int main(int argc, char **argv) {
+    int a[3];
+    return _Countof(a);
+}" HAVE_COUNTOF)
+
 if(PUTTY_DEBUG)
   add_compile_definitions(DEBUG)
 endif()

misc.h

@@ -353,8 +353,12 @@ const char *conf_id(int key);
 #endif
 
 #ifndef lenof
+#if HAVE_COUNTOF
+#define lenof(x) _Countof(x)
+#else
 #define lenof(x) ( (sizeof((x))) / (sizeof(*(x))))
 #endif
+#endif
 
 #ifndef min
 #define min(x,y) ( (x) < (y) ? (x) : (y) )

utils/tree234.c

@@ -1361,7 +1361,13 @@ int mycmp(void *av, void *bv)
     return strcmp(a, b);
 }
 
-#define lenof(x) ( sizeof((x)) / sizeof(*(x)) )
+#ifndef lenof
+#if HAVE_COUNTOF
+#define lenof(x) _Countof(x)
+#else
+#define lenof(x) ( (sizeof((x))) / (sizeof(*(x))))
+#endif
+#endif
 
 char *strings[] = {
     "a", "ab", "absque", "coram", "de",