improve platform compatibility

ext/decoders/native/astc.c

@@ -54,7 +54,7 @@ static inline int getbits(const uint8_t *buf, const int bit, const int len) {
 }
 
 static inline uint_fast64_t getbits64(const uint8_t *buf, const int bit, const int len) {
-    uint_fast64_t mask = len == 64 ? -1 : (1ull << len) - 1;
+    uint_fast64_t mask = len == 64 ? 0xffffffffffffffff : (1ull << len) - 1;
     if (len < 1)
         return 0;
     else if (bit >= 64)

ext/decoders/native/extconf.rb

@@ -1,5 +1,6 @@
 require 'mkmf'
 
+append_cppflags('-std=c11')
 append_cppflags('-O3')
 append_cppflags('-Wall')
 append_cppflags('-Wextra')