avutil/file_open: Fix build error with wasi

Don't assume tempnam is available when !HAVE_MKSTEMP. Check tempnam
explicitly in configure.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

configure

@@ -2414,6 +2414,7 @@ SYSTEM_FUNCS="
     sysconf
     sysctl
     sysctlbyname
+    tempnam
     usleep
     UTGetOSTypeFromString
     VirtualAlloc
@@ -6554,6 +6555,7 @@ check_struct "sys/stat.h" "struct stat" st_mtim.tv_nsec -D_BSD_SOURCE
 check_func  strerror_r
 check_func  sysconf
 check_func  sysctl
+check_func  tempnam
 check_func  usleep
 
 check_func_headers conio.h kbhit

libavutil/file_open.c

@@ -112,7 +112,10 @@ int avpriv_tempfile(const char *prefix, char **filename, int log_offset, void *l
 {
     FileLogContext file_log_ctx = { &file_log_ctx_class, log_offset, log_ctx };
     int fd = -1;
-#if !HAVE_MKSTEMP
+#if HAVE_MKSTEMP
+    size_t len = strlen(prefix) + 12; /* room for "/tmp/" and "XXXXXX\0" */
+    *filename  = av_malloc(len);
+#elif HAVE_TEMPNAM
     void *ptr= tempnam(NULL, prefix);
     if(!ptr)
         ptr= tempnam(".", prefix);
@@ -120,8 +123,7 @@ int avpriv_tempfile(const char *prefix, char **filename, int log_offset, void *l
 #undef free
     free(ptr);
 #else
-    size_t len = strlen(prefix) + 12; /* room for "/tmp/" and "XXXXXX\0" */
-    *filename  = av_malloc(len);
+    return AVERROR(ENOSYS);
 #endif
     /* -----common section-----*/
     if (!*filename) {