Use Homebrew libyaml if it is available

This checks for Homebrew's yaml to see if it
is available and that `--with-libyaml-dir` hasn't
been specified.

Related issue: #379
Merge master into docwhat-homebrew-yaml

Conflicts:
	share/ruby-build/1.9.1-p378
	share/ruby-build/1.9.1-p430
	share/ruby-build/1.9.2-p0
	share/ruby-build/1.9.2-p180
	share/ruby-build/1.9.2-p290
	share/ruby-build/1.9.2-p318
	share/ruby-build/1.9.2-p320
	share/ruby-build/1.9.3-p0
	share/ruby-build/1.9.3-p125
	share/ruby-build/1.9.3-p194
	share/ruby-build/1.9.3-p286
	share/ruby-build/1.9.3-p327
	share/ruby-build/1.9.3-p362
	share/ruby-build/1.9.3-p374
	share/ruby-build/1.9.3-p385
	share/ruby-build/1.9.3-p392
	share/ruby-build/1.9.3-p429
	share/ruby-build/1.9.3-p448
	share/ruby-build/1.9.3-preview1
	share/ruby-build/1.9.3-rc1
	share/ruby-build/2.0.0-preview1
Have Rubinius 2 use libyaml from Homebrew if available
Simplify stubbing commands in tests

No more awkward juggling of PATH entries and preset stubs in version
control. Commands are stubbed by symlinking the main stub from a
temporary `$TMP/bin` directory that's added once to PATH in tests.
Prevent glob expansion in stub patterns

When stub patterns were broken down into an array, they were
accidentally subject to glob expansion. E.g., a pattern '*' might expand
to whatever is in the current directory. This is unwanted, as we need to
preserve patterns as-is.
Add tests for Homebrew libyaml integration
Merge branch 'docwhat-homebrew-yaml'

Use Homebrew libyaml if available.

Closes #381