dln_find.c: define S_ISREG

* dln_find.c (dln_find_1): on some platforms, S_ISREG() is not
  provided.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49396 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

dln_find.c

@@ -278,6 +278,9 @@ dln_find_1(const char *fname, const char *path, char *fbuf, size_t size,
 	}
 #endif /* _WIN32 or __EMX__ */
 
+#ifndef S_ISREG
+# define S_ISREG(m) (((m) & S_IFMT) == S_IFREG)
+#endif
 	if (stat(fbuf, &st) == 0 && S_ISREG(st.st_mode)) {
 	    if (exe_flag == 0) return fbuf;
 	    /* looking for executable */