RFR 8u40: 8062635: Enable custom CompileJavaClasses.gmk

[roger riggs]

Please review a change to include CompileJavaClasses.gmk from the 
CUSTOM_MAKE_DIR.

Please verify the change from ":=" to "+=" at line 61,  it was 
overwriting instead of appending.

Webrev:
    http://cr.openjdk.java.net/~rriggs/webrev-custom-compile-java/

Issue:
    https://bugs.openjdk.java.net/browse/JDK-8062635

Thanks, Roger