Update mkmf from MRI 2.0
Use same directory for arch files
Add UNREACHABLE to configure
Update to OpenSSL extension from 2.0
Add thread.h dummy header, used by OpenSSL
Use rb_funcall over RHASH
Don't support rb_*_cloexec for now
Add rb_thread_call_without_gvl and friends for 2.0 mode
Use rb_ary_entry over RARRAY_PTR in OpenSSL
Fix spec for sitelibdir location
Remove Apple OpenSSL check for now
Remove accidentally added Makefile
Add back deprecation warning suppression
Remove SITE_PATH usage

This was referring to a non existant path anyway. We now just point to
the default header directory for extensions that need this through
RbConfig.
Add specs for rb_thread_call_without_gvl(2)
Setup gitignore for openssl extension in 2.0
Reapply thread safety patch to OpenSSL extension
Performance improvements by better handling ascii_only updates

This improves the propagation of ascii_only status so we can use faster
mechanisms for example for byte based substringing. This also makes
sure that we setup empty strings with an ascii compatible encoding as
ascii only, which makes sure it works for appending where we start with
an empty string.

This happens for example in Array#join and string building. Some
informal benchmarks when generating RDoc when installing the FFI
extension:

Before:

Fetching: ffi-1.9.0.gem (100%)
Building native extensions.  This could take a while...
Successfully installed ffi-1.9.0
Parsing documentation for ffi-1.9.0
Installing ri documentation for ffi-1.9.0
Done installing documentation for ffi after 176 seconds
1 gem installed

real    3m8.430s
user    4m51.329s
sys     0m5.908s

After:

Fetching: ffi-1.9.0.gem (100%)
Building native extensions.  This could take a while...
Successfully installed ffi-1.9.0
Parsing documentation for ffi-1.9.0
Installing ri documentation for ffi-1.9.0
Done installing documentation for ffi after 23 seconds
1 gem installed

real    0m34.286s
user    0m46.962s
sys     0m9.238s