JDK9: RFR: 8019470: Changes needed to compile JDK8 on MacOS with clang compiler

[Henry Jen]

Hi,

Please review a webrev to enable XCode5 build on Mac OS X, this webrev 
add basic clang flags and detect Xcode version to set default toolchain 
to clang for Xcode5 and after, otherwise set to gcc for Mac OS X.

http://cr.openjdk.java.net/~henryjen/jdk9/8019470/1/webrev/

This patch build on Maverick with Xcode 5, but is not yet sufficient to 
enable clang build on my Arch Linux, where I installed clang 3.4. There 
are some undeclare identifier|function|type warnings/errors, the cause 
of which is still under investigation, my guess lies in precompile 
header, but I think that can be addressed later.

Cheers,
Henry