[ruby/prism] Fallthrough fallback for unknown compilers

ruby/prism@cc810b1e06

Author: kddnewton

prism/defines.h

@@ -253,6 +253,8 @@
     #define PRISM_FALLTHROUGH __attribute__((fallthrough));
 #elif defined(_MSC_VER)
     #define PRISM_FALLTHROUGH __fallthrough;
+#else
+    #define PRISM_FALLTHROUGH
 #endif
 
 #endif