[Vendor] Allow the user to override default cflags.
[rake] Re-instate top-level `clean` task and refactor into XcodeConfig.

Related to:
* https://github.com/lrz/RubyMotion/commit/646cbb1918a1796ad595082e1887e80c6c8c7f24
* https://github.com/lrz/RubyMotion/commit/bddc880f9868af26285a6266403885f86803c746
[Command::Update] Support Axel for optimised downloading.
[Config] Capture and work with the OS X host version in a proper way.

Fixes http://hipbyte.myjetbrains.com/youtrack/issue/RM-633
[Config] Get rid of another `sw_vers` use.
[XcodeConfig] Fix missing space in certificate error message.
[CodeSign] Fix YARD documentation type annotations.
[OSX] Sign embedded frameworks individually.

Fixes http://hipbyte.myjetbrains.com/youtrack/issue/RM-642
[OSX] Refactor codesigning code from last commit.
Some small YARD doc fixes.
recognize Provisioning Profile which has "iOSTeam Provisioning..."

http://hipbyte.myjetbrains.com/youtrack/issue/RM-643
[vendor] Only ever try to run `xcodebuild clean` if the dir really exists.
make 'motion --version' print both stable and pre-release versions
[clean] Do not remove `momd` when no `xcdatamodeld` is available.

Related to http://hipbyte.myjetbrains.com/youtrack/issue/RM-649
[Config] Move ivar declaration to file where it's actually used.
not embed kernel-*.bc into each compiled files

http://hipbyte.myjetbrains.com/youtrack/issue/RM-652
enable arm64 again, remove armv7s
make sure the builder archs does not contain duplicates
revert enabling arm64 by default
2.38