RFR 8233880: Support compilers with multi-digit major version numbers

[Tim Bell]

Florian:

> The current regular expression only matches one digit before the first
> dot.
> 
> Bug: <https://bugs.openjdk.java.net/browse/JDK-8233880>
> Webrev: <http://cr.openjdk.java.net/~fweimer/8233880/webrev.01/>
> 
> Tested against GCC 10.0.0.  configure no longer prints a C/C++ version
> mismatch warning.

Looks good.

Tim