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

Upgrades rubygems for 1.9.1: 1.3.5 -> 1.3.7

Ref. #290, #311

Build a shared openssl to link to Ruby as a stopgap to fix build issues some people are having, pending understanding why static linking isn't working.

fix exit status of install with verbose

bump versions to 2.1.0

$ strings ~/.rbenv/versions/2.0.0-rc2/openssl/lib/libcrypto.a |
  grep -B3 SSL_CERT_FILE
/Users/jeremy/.rbenv/versions/2.0.0-rc2/openssl/ssl/certs
/Users/jeremy/.rbenv/versions/2.0.0-rc2/openssl/ssl/cert.pem
SSL_CERT_DIR
SSL_CERT_FILE

Added a definition for 2.0.0-p0