Provide function that identifies compiler to patch-level

Some distributions modify -dumpversion so that it only returns major
and minor numbers, not the patch level. This is usually OK for
determining features, but when debugging the output log, we really
want the full number.
Print compiler version number when configuring
Update regular expression to handle more variants

Older CentOS versions returns the version string on this format:

    gcc (aaa) x.y.z yyyymmdd (bbb)
Merge pull request #180 from rolk/180_compver

Diagnose compiler version in configuration output