Don't redefine alloca

src/libsodium/sodium/utils.c

@@ -23,19 +23,21 @@
 
 #if HAVE_ALLOCA_H
 # include <alloca.h>
-#elif defined __GNUC__
+#elif !defined(alloca)
+# if defined(__GNUC__)
 #  define alloca __builtin_alloca
-#elif defined _AIX
+# elif defined _AIX
 #  define alloca __alloca
-#elif defined _MSC_VER
+# elif defined _MSC_VER
 #  include <malloc.h>
 #  define alloca _alloca
-#else
+# else
 #  include <stddef.h>
 #  ifdef  __cplusplus
 extern "C"
 #  endif
 void *alloca (size_t);
+# endif
 #endif
 
 #include "core.h"