Add a separate check for [v]asprintf() instead of checking for _GNU_SOURCE

Platforms like i686-pc-mingw32 defines _GNU_SOURCE but does not contain
the functions.
Signed-off-by: Timothy Gu <timothygu99@gmail.com>

configure.ac

@@ -110,6 +110,27 @@ if test "${ac_cv_cpp_variadic_macros}" != "no"; then
     AC_DEFINE(HAVE_VARIADIC_MACROS, 1, Support for variadic macros)
 fi
 
+dnl Check for asprintf(). Systems with asprintf() must also have vasprintf(),
+dnl so it is not checked separately.
+AC_CACHE_CHECK([for asprintf()],
+    [ac_cv_asprintf],
+    [AC_COMPILE_IFELSE([
+        AC_LANG_SOURCE([[
+            #include <stdio.h>
+            #include <stdlib.h>
+            int main(void) {
+                char *text = NULL;
+                int ret = asprintf(&text, "test");
+                if (ret >= 0) free(text);
+                return 0;
+            }
+        ]])],
+        ac_cv_asprintf=yes,
+        ac_cv_asprintf=no)])
+if test "${ac_cv_asprintf}" != "no"; then
+    AC_DEFINE(HAVE_ASPRINTF, 1, [Support for asprintf() and vasprintf()])
+fi
+
 dnl
 dnl Generate Makefiles and other output files
 dnl

src/dvbpsi.c

@@ -508,7 +508,7 @@ bool dvbpsi_packet_push(dvbpsi_t *p_dvbpsi, uint8_t* p_data)
  *  1 is warning and errors
  *  2 is debug, warning and errors
  *****************************************************************************/
-#if !defined(_GNU_SOURCE)
+#if !defined(HAVE_ASPRINTF)
 #   define DVBPSI_MSG_SIZE 1024
 #endif
 
@@ -523,7 +523,7 @@ void dvbpsi_message(dvbpsi_t *dvbpsi, const dvbpsi_msg_level_t level, const char
         va_list ap;
         va_start(ap, fmt);
         char *msg = NULL;
-#if defined(_GNU_SOURCE)
+#if defined(HAVE_ASPRINTF)
         int err = vasprintf(&msg, fmt, ap);
 #else
         msg = malloc(DVBPSI_MSG_SIZE);
@@ -544,7 +544,7 @@ void dvbpsi_message(dvbpsi_t *dvbpsi, const dvbpsi_msg_level_t level, const char
 #else
 
 /* Common code for printing messages */
-#   if defined(_GNU_SOURCE)
+#   if defined(HAVE_ASPRINTF)
 #       define DVBPSI_MSG_COMMON(level)                         \
     do {                                                        \
         va_list ap;                                             \