RFR: JDK-8062159: Fix Xrender check to work with sysroot

[Tim Bell]

Erik:

> Please review this small fix. In configure, there is a check on 
> Solaris 10 for the definition of XLinearGradient in Xrender.h. The 
> check is crude in that it's just a plain grep. It also isn't checking 
> the correct file if using a devkit/sysroot. So far, the build has 
> still worked, but when I recently experimented with upgrading the 
> sysroot to later versions of Solaris, it failed because this check 
> returned the wrong answer.
>
> I have converted the check to an AC_COMPILE_IFELSE call and verified 
> that it gives the correct answers with Solaris 10u6, 10u10 and 11u1 
> headers.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8062159
> Webrev: http://cr.openjdk.java.net/~erikj/8062159/webrev.root.01/

Looks good to me-

Tim