Fix compilation errors for c99

ENUM_OVER_INT is sometimes not defined. Use #ifdef instead if #if.

include/ruby/internal/fl_type.h

@@ -217,7 +217,7 @@ ruby_fl_type {
     RBIMPL_FL_USER_N(16),
     RBIMPL_FL_USER_N(17),
     RBIMPL_FL_USER_N(18),
-#if ENUM_OVER_INT
+#ifdef ENUM_OVER_INT
     RBIMPL_FL_USER_N(19),
 #else
 # define RUBY_FL_USER19 (RBIMPL_VALUE_ONE<<(RUBY_FL_USHIFT+19))