Added JRuby 1.7.4

Replaced 1.9.3-p426 with 1.9.3-p429 definition (p426 was yanked after release due to build issues)

1.9.3-p426 was deprecated by 1.9.3-p429 since it won't build on RHEL6
systems (and others?)

added 1.9.3-p426 and 2.0.0-p195

Allow define RUBY_CONFIGURE_OPTS on rbx configure

The configure of rbx can take a RUBY_CONFIGURE_OPTS. You can by example
use the `--skip-prebuilt` OPTS

Restore -O3 default when we build with clang

Unable to clone ruby-build from behind firewall

Fixes #267

Fixes `rbenv install` in a directory with a .ruby-version file that specifies ree

* Decrease likelihood of cache collisions
* Avoid file:// urls (and therefore the need to escape)

if RUBY_BUILD_CACHE_PATH is set, create and maintain a local bare git
repository in this location.  The name of the git repository is based
on the git URL, so it will be shared between branches.

add topaz-dev

Added mruby

Fix build failure when clang installed and not used

The previous version of this logic was causing us to
pass the flag when
 * clang is on the PATH, and
 * $CC, or 'clang' if CC unset, accepts the flag.
But this is totally wrong if we have clang installed,
haven't set $CC, and are going to end up using gcc.
We end up passing the flag to gcc, which rejects it.

The real fix is to put this in the autoconf goo in MRI
upstream -- the only correct way to decide whether to pass
this flag is after we know exactly what compiler we're using
and can test if that compiler accepts the flag.  But we can
do better than before by approximating autoconf's choice of
compiler as $CC if set, 'cc' otherwise (which will typically
be a symlink to gcc or clang or another.)

Fixes: https://github.com/sstephenson/ruby-build/issues/319
  and #325, which is a dupe.

add jruby-1.7.4-dev

Keep downloaded tarball sometimes