* configure.ac: introduce new configure option `--enable-mjit` and
  `--disable-mjit`. Default is "enable".
  `--disable-mjit` disables all of MJIT features so that `ruby --jit`
  can't enable MJIT.
  This option affect a macro `USE_MJIT`.
  This change remove `--enable/disable-install-mjit-header` option.

* Makefile.in: introduce the `ENABLE_MJIT` variable.

* common.mk: use `ENABLE_MJIT` option.

* internal.h: respect `USE_MJIT`. Same as other *.c, *.h.

* test/ruby/test_jit.rb: check `ENABLE_MJIT` key of rbconfg.rb.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65204 b2dd03c8-39d4-4d8f-98ff-823fe69b080e