RFR: 8057751: CompileNativeLibraries for custom build

[roger riggs]

Please review for 8u40 this improvement to allow delegation
of building native libraries to the custom/closed build.

Issue: JDK-8057751 <https://bugs.openjdk.java.net/browse/JDK-8057751> 
CompileNativeLibraries for custom build

Webrev: http://cr.openjdk.java.net/~rriggs/webrev-native-lib-8057751/ 
<http://cr.openjdk.java.net/%7Erriggs/webrev-native-lib-8057751/>

Thanks, Roger