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_bsdlike.c

@@ -1,5 +1,8 @@
+#define _GNU_SOURCE // to obtain asprintf/vasprintf
 #include "platform.h"
 
+#include <stdio.h>
+
 #include <err.h>
 #include <stdarg.h>
 
@@ -12,6 +15,20 @@
 #include <sys/resource.h>
 #endif
 
+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 arg)
+{
+  return vasprintf(strp, fmt, arg);
+}
+
 void h_platform_errx(int err, const char* format, ...) {
   va_list ap;
   va_start(ap, format);
@@ -62,5 +79,5 @@ int64_t h_platform_stopwatch_ns(struct HStopWatch* stopwatch) {
 
   // time_diff is in ns
   return (ts_now.tv_sec - stopwatch->start.tv_sec) * 1000000000
-	  + (ts_now.tv_nsec - stopwatch->start.tv_nsec);
+          + (ts_now.tv_nsec - stopwatch->start.tv_nsec);
 }