ruby.c: initialize extra optional extensions linked statically

Add a hook point to initialize extra extension libraries. The default
hook function is replaced when linking a strong `Init_extra_exts`
symbol. A builder can insert an object file that defines Init_extra_exts
by XLDFLAGS.

ruby.c

@@ -1524,6 +1524,15 @@ ruby_init_prelude(void)
 
 void rb_call_builtin_inits(void);
 
+// Initialize extra optional exts linked statically.
+// This empty definition will be replaced with the actual strong symbol by linker.
+#if RBIMPL_HAS_ATTRIBUTE(weak)
+__attribute__((weak))
+#endif
+void Init_extra_exts(void)
+{
+}
+
 static void
 ruby_opt_init(ruby_cmdline_options_t *opt)
 {
@@ -1542,6 +1551,7 @@ ruby_opt_init(ruby_cmdline_options_t *opt)
     rb_warning_category_update(opt->warn.mask, opt->warn.set);
 
     Init_ext(); /* load statically linked extensions before rubygems */
+    Init_extra_exts();
     rb_call_builtin_inits();
     ruby_init_prelude();