libcpp: Complete the COUNTER support with upstream implementation.

We brought an original COUNTER implementation in r228474, however, it
was missing documentation and it had a different behaviour for precompiled
headers with respect to the upstream version. Since the upstream version
is under the same license as GCC4.2, bring the missing pieces to reduce
differences against upstream.

Optained from: GCC pre-4.3 (rev. 125041 ; GPLv2)