mjit.c: another try for fixing solaris build

Reverted r62192 and added another try.
I prepared Solaris environment but I couldn't reproduce the RubyCI build
failure. So I'm testing on trunk.

I found the following article and it fixed the same problem with passing
`-D__EXTENSIONS__`. So this commit defines `__EXTENSIONS__` for Solaris.
https://bitbucket.org/ged/ruby-pg/issues/236/compile-error-usr-include-signalh-92-1

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62204 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

mjit.c

@@ -78,7 +78,10 @@
 #include <dlfcn.h>
 #endif
 
-#include "internal.h"
+#ifdef __sun
+#define __EXTENSIONS__ 1
+#endif
+
 #include "vm_core.h"
 #include "mjit.h"
 #include "version.h"