Fix build scripts.

Fix memory leaks in mirb.

This will improve the default performance of mruby; fix #4045

Export mrb_utf8_from_locale() and mrb_locale_from_utf8()

Stop to use freed value

Fix wrong free function

mrb_locale_free() should be used for the return value of
mrb_locale_from_utf8().

`mrb_raisef()` only takes `%S` specifier. If you don't have extra
arguments, use `mrb_raise()`.

incorporate fixes from iij/socket

If your system has sa_len but is not BSD-derived, define HAVE_SA_LEN=1
on mrbgem.rake or build_config.rb.

When we have "struct sockaddr_un *s_un", we could not assume *s_un
points to a memory region which size is at least sizeof(*s_un).
Even worse, it may be shorter than sizeof(struct sockaddr) on some
systems.

ref: https://github.com/iij/mruby-socket/issues/45

Ranges are almost immutable but `initialize` and `initialize_copy`
modify the receiver so that we need to add a write barrier.

The problem was caused by `Fiber.current.resume'.

Fix unnecessary rebuild of tests.

This reverts commit c6736357.
The assumption was wrong and caused the issue; fix #4020

Copying all flags from the original object may overwrite the clone's
flags e.g. the embedded flag.

[mruby-socket] Declare inet_ntop and inet_pton as static

fix broken lexical variables

This change is required to support #4038.