set --with-gcc=gcc-4.2 on lion + xcode 4.1

Include --with-gcc config flag for gcc-4.2 on Lion, per https://github.com/ruby/ruby/commit/9151ed22b360ba3d2bde3b51969a682f9a4a3096

Add install_git and refactor to support it. Pass package name, git url, git ref, and build commands.

rbenv-install

This reverts commit 20f7b5c0.

Conflicts:

	bin/ruby-build

REE should use the do not install helpful gems option

no need to start with a bunch of cruft we may not even be wanting to use, let users install their gems themselves

Add definition for jruby-1.6.4

Could you add this definition to ruby-build ? Thanks!

Added definition for Ruby 1.9.1-p378 (rubygems 1.3.5) - The standard debian versions for Ubuntu Lucid 10.04 (which we still run on production... for a little while longer)

Add definition for Ruby 1.8.7-p249.

Added support to platforms where bash is not in /bin directory 

make bin/ruby-build to executable

Solaris doesn't come with readlink so use greadlink if available instead.

Solaris doesn't come with readlink so use greadlink if available instead. (Taken from Ryan Tomayko's "GNU is killing Solaris", c.f. http://tomayko.com/writings/gnu-is-killing-solaris)