util: Provide a secure_getenv fallback for platforms without it

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27381>

meson.build

@@ -1378,6 +1378,7 @@ functions_to_detect = {
   'getrandom': '',
   'qsort_s': '',
   'posix_fallocate': '',
+  'secure_getenv': '',
 }
 
 foreach f, prefix: functions_to_detect

src/util/u_debug.h

@@ -39,6 +39,7 @@
 #define U_DEBUG_H_
 
 #include <stdarg.h>
+#include <stdlib.h>
 #include <string.h>
 #if !defined(_WIN32)
 #include <sys/types.h>
@@ -403,6 +404,13 @@ __normal_user(void)
 #endif
 }
 
+#ifndef HAVE_SECURE_GETENV
+static inline char *secure_getenv(const char *name)
+{
+   return getenv(name);
+}
+#endif
+
 #define DEBUG_GET_ONCE_BOOL_OPTION(sufix, name, dfault) \
 static bool \
 debug_get_option_ ## sufix (void) \