RFR: JDK-8199224 Refactor add_native_source in SetupNativeCompilation

Wed Mar 7 14:02:27 UTC 2018

The function add_native_source has been with us since the original 
version of the build-infra system. It needs to be adapted to modern 
standards, and to have the flag setting unified so it happens in only 
one place.

I have verified using BUILD_COMPARE that this does not affect the 
generated output.

Bug: https://bugs.openjdk.java.net/browse/JDK-8199224
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8199224-refactor-add_native_source/webrev.01

/Magnus