8210423: Backport of 8034788 breaks GCC version detection

[Severin Gehwolf]

On Wed, 2018-09-05 at 17:05 +0100, Andrew Haley wrote:
> The JDK8u backport of 8034788 changed the variable name CXX_VERSION
> (in configure) to CXX_VERSION_NUMBER. Unfortunately this was not done
> consistently, and as a result the version number check for GCC is
> broken. As a result of that brokenness, HotSpot built on all GCC
> versions >=6 is unstable.
> 
> Intriguingly, the comments aroud te affected code are correct, and
> refer to CXX_VERSION_NUMBER, but the actual variable name is wrong.
> 
> http://cr.openjdk.java.net/~aph/8210423/
> 
> OK?

This fixes the issue I was seeing on jdk8u-dev head on F28 with GCC
8.1.1. java -version crashed on release build. That's a +1 from me (not
a Reviewer).

Thanks,
Severin