Introduce {v,}asprintf for windows

We have now again two functions called h_platform_vasprintf and
h_platform_asprintf. On windows they are implemented in terms of vsnprint
and the like. On BSD/GNU libraries we use the supplied vasprintf and
asprintf.

src/platform_win32.c

@@ -1,7 +1,46 @@
 #include "platform.h"
 
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
 #define WIN32_LEAN_AND_MEAN
-#include "windows.h"
+#include <windows.h>
+
+int h_platform_asprintf(char**strp, const char *fmt, ...)
+{
+  va_list ap;
+  va_start(ap, fmt);
+  int res = h_platform_vasprintf(strp, fmt, ap);
+  va_end(ap);
+  return res;
+}
+
+int h_platform_vasprintf(char**strp, const char *fmt, va_list args)
+{
+  va_list ap;
+  va_copy(ap, args);
+  int non_null_char_count = _vscprintf(fmt, ap);
+  va_end(ap);
+
+  if (non_null_char_count < 0) {
+    return -1;
+  }
+
+  size_t buffer_size = 1 + non_null_char_count;
+  char* buffer = malloc(buffer_size);
+
+  va_copy(ap, args);
+  int res = vsnprintf_s(buffer, buffer_size, non_null_char_count, fmt, ap);
+  if (res < 0) {
+    free(buffer);
+  } else {
+    buffer[non_null_char_count] = 0;
+    *strp = buffer;
+  }
+  va_end(ap);
+
+  return res;
+}
 
 void h_platform_errx(int err, const char* format, ...) {
   // FIXME(windows) TODO(uucidl): to be implemented