If a commit introduced a new Ruby definition, we want it mirrored even
if there might have been an accidental failure on CI.

Fixes https://travis-ci.org/sstephenson/ruby-build/builds/13131225

Fixes #148

Because OS X Mountain Lion removed X Windows, compiling Ruby 1.8 would
fail unless the user installed XQuartz manually and passed:

    CPPFLAGS=-I/opt/X11/include rbenv install 1.8.7-p374

This auto-detects if `/opt/X11/include` is present on the system and
configures CPPFLAGS accordingly. However if XQuartz was never installed,
we simply configure Ruby using `--without-tk`.

Fixes #193 #207

References 35324692

Alt. implementation of #401, #434

The problem wasn't in quoting as per 0b520617, but in the fact that
Ruby trunk added a LDFLAGS checker that aborts if any of the paths
listed in it are missing:
https://github.com/ruby/ruby/commit/3636f8c0f56ddf15e26e28e7a38e748588fad976

This is probably a bug in Ruby, but for now a simple workaround is to
iterate through paths in LDFLAGS and ensure they exist.

References #441

Stop quoting in LDFLAGS and CPPFLAGS. Ruby trunk chokes on it:
https://github.com/ruby/ruby/commit/3636f8c0f56ddf15e26e28e7a38e748588fad976

Closes #441

Example:

        export RAKE_STUB_DEBUG=2

Some Rubies need Rake or Bundler for the installation process. However,
since the host Ruby version might not have those gems installed, install
them in a temporary GEM_HOME and add their executables to PATH.

Fixes #426

Rake might not be available on the system, but if there is a Gemfile,
assume that it bundles rake.

Fixes #440

Use the `cp -f` flag to force unlinking non-writable files.

Closes #384

When installing mruby-dev, the cp command doesn't use the force flag.
This causes an issue when using a custom build configuration (via
MRUBY_CONFIG environment variable) and installing mrbgems from Github,
as the pack files in the git repositories are read-only.

Helps avoid the pitfall where ERR trap has no effect.
See 262eb165

Previously, if `http get` failed, the `download_tarball` function would
still continue since the ERR trap had no effect at that point.

Given a script in the form of `{ ... } || return 1`, the expressions
that are the part of the first group are not subject to ERR trap since
they are non-last in a chain of expressions.

However, since we still can't count on the ERR trap taking effect in
this phase, better just rewrite the function to manually abort.

Fixes #394

Closes #293

Fixes #110

- use cache path feature to skip HTTP fetch atogether
- unified build.log

* Shorter.
* Preserve timestamps and permissions.
* Overwrite existing files.

`cd` into project's root on every invocation.

Fixes #243

Works on OS X but fails on CI.

Use Homebrew libyaml if available.

Closes #381