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

[Erik Joelsson]

Hello,

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/

/Erik