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

[Florian Weimer]

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.

Thanks,
Florian