xkbvalidate: Don't rely on GNU extensions

The only reason why I was using _GNU_SOURCE was because of vasprintf(),
so getting rid of that extension should make the source way more
portable.

When using vsnprintf() with a null pointer for the output buffer and a
size of 0, I wasn't quite sure whether this would be undefined
behaviour, so I looked it up in the C11 standard.

In section 7.21.6.5, it explicitly mentions this case, so we're lucky:

If n is zero, nothing is written, and s may be a null pointer.

Additionally, section 7.21.6.12 writes the following about vsnprintf():

The vsnprintf function does not invoke the va_end macro.

So to be sure to avoid undefined behaviour I subsequently added the
corresponding va_end() calls.

With this, the platforms attribute is now "unix", because the program
should now even run on OS X, even though it usually wouldn't be needed.

Signed-off-by: aszlig <[e-mail]>

pkgs/tools/X11/xkbvalidate/default.nix

···
   meta = {
     description = "NixOS tool to validate X keyboard configuration";
     license = lib.licenses.mit;
-    platforms = lib.platforms.linux;
+    platforms = lib.platforms.unix;
     maintainers = [ lib.maintainers.aszlig ];
   };
 } ''
   mkdir -p "$out/bin"
-  $CC -std=gnu11 -Wall -pedantic -lxkbcommon ${./xkbvalidate.c} \
+  $CC -std=c11 -Wall -pedantic -lxkbcommon ${./xkbvalidate.c} \
     -o "$out/bin/validate"
 ''

pkgs/tools/X11/xkbvalidate/xkbvalidate.c

···
-#define _GNU_SOURCE
 #include <stdarg.h>
 #include <stdbool.h>
 #include <stdio.h>
···
 static void add_log(struct xkb_context *ctx, enum xkb_log_level level,
                     const char *fmt, va_list args)
 {
+    size_t buflen;
+    va_list tmpargs;
+
     log_buffer_size++;
 
     if (log_buffer == NULL)
···
         return;
     }
 
-    if (vasprintf(&log_buffer[log_buffer_size - 1], fmt, args) == -1) {
+    /* Unfortunately, vasprintf() is a GNU extension and thus not very
+     * portable, so let's first get the required buffer size using a dummy
+     * vsnprintf and afterwards allocate the returned amount of bytes.
+     *
+     * We also need to make a copy of the args, because the value of the args
+     * will be indeterminate after the return.
+     */
+    va_copy(tmpargs, args);
+    buflen = vsnprintf(NULL, 0, fmt, tmpargs);
+    va_end(tmpargs);
+
+    log_buffer[log_buffer_size - 1] = malloc(++buflen);
+
+    if (vsnprintf(log_buffer[log_buffer_size - 1], buflen, fmt, args) == -1) {
         perror("log line alloc");
         log_alloc_success = false;
-        return;
     }
+    va_end(args);
 }
 
 static void print_logs(void)