Add patch to allow build with clang 7.3

[aviat]

This adds the -Wno-shift-negative-value flag to Clang. Clang 7.3 was introduced
in OSX with XCode 7.3.

Without this flag, compiling with Clang 7.3 fails with errors such as:

In file included from ../src/elements.h:32:
../src/objects.h:5252:44: error: shifting a negative signed value is undefined
[-Werror,-Wshift-negative-value]
  static const int kElementsKindMask = (-1 << kElementsKindShift) &
                                        ~~ ^
../src/objects.h:7386:36: error: shifting a negative signed value is undefined
[-Werror,-Wshift-negative-value]
      (~kMaxCachedArrayIndexLength << kArrayIndexHashLengthShift) |
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
2 errors generated.

[aviat]

This should resolve #205, as well as:

• Unable to install on OS X 10.11.4 with Ruby 2.2.4 therubyracer#405
• Unable to compile dependency libv8 after upgrading to XCode 7.3 therubyracer#403

[julik]

@cowboyd it would be great if this could be merged, I already had to install_if on 3 projects that use libv8 indirectly (I know it is a +1 comment at heart and I will be punished, and yet hope dies last).

[ignisf]

Thank you for the contribution!

[aviat]

You are welcome @ignisf. Any idea when the new gem could be published?

[ignisf]

I usually leave releasing the gem to @cowboyd, as I have no way of producing osx binaries and it's a bad idea to leave all osx users to deal with the touchy build process of v8. So... No, no idea 😕...

[aviat]

@ignisf I could give you the darwin14 / darwin15 builds if that helps!

[ignisf]

Thank you, but I would not publish binaries made on a system I can't trust.

[aviat]

Which is a relief for all of us :) So let's wait for @cowboyd.