RFR: JDK-8073498: Enhance GensrcProperties.gmk to allow an alternative source root

[Tim Bell]

Erik:

> Please review these two small fixes to the build system. When we tried 
> to add a custom Gensrc-<module>.gmk file, it didn't work as expected. 
> The -I flags on the make command line were wrong, so it couldn't 
> import the GensrcCommon.gmk or GensrcProperties.gmk from 
> jdk/make/gensrc. Also, GensrcProperties.gmk did not support generating 
> java files from properties in a different source root than 
> jdk/src/<module>.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8073498
> Webrev: http://cr.openjdk.java.net/~erikj/8073498/webrev.01/
>
> These changes have been tested with the patch for JDK-8062750, through 
> which we discovered these shortcomings.

Looks good to me.

/Tim