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

[Erik Joelsson]

Hello,

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/

/Erik