RFR: JDK-8138694: Devkit build on Macosx still requires Xcode to be installed

[Tim Bell]

Erik:

> While trying the macosx build using devkits again, I happened to be on 
> a system where some Xcode install/update error had left the 
> /usr/include directory missing. This shouldn't be an issue since we 
> have all the necessary include files in the SDK inside the 
> devkit/Xcode application.
>
> Getting this to work required several changes and cleanups that will 
> actually improve build consistency overall. It will also guarantee 
> that we use the correct system headers in all cases.
>
> * Setup SYSROOT_*FLAGS before detecting toolchain and make those flags 
> available to the configure toolchain tests.
> * Setup the user supplied extra flags before toolchain detection so 
> that those are also used during configure toolchain tests.
> * When not cross compiling, use the SYSROOT_*FLAGS also when compiling 
> build tools, like adlc in hotspot, and gensrc tools in jdk.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8138694
> Webrev: http://cr.openjdk.java.net/~erikj/8138694/webrev.top.01 

Looks good to me.

/Tim