tests: fix a failure in Lex/C++ tests on Mac OS X

tests: fix a failure in Lex/C++ tests on Mac OS X

The <cstdio> header from Xcode-4.3.2 on Mac OS X 10.7.3 declares a 'isatty'
function with C++ linkage, that conflicts with our dummy definition of the
same function, which in turn is required to work around the absence of the
unistd.h header on MinGW (see commit 'v1.11-2138-gfeea090' of 11-04-2012).
So we tweak the affected tests to work around this new problem as well.

This fixes automake bug#11345.

* t/lex-clean-cxx.sh ($required): Since we are at it, add an explicit
'c++' requirement.
* t/lex-depend-cxx.sh ($required): Likewise.
(my-hdr.hxx): Don't include <cstdio>.  The "using namespace" directive
should still be enough to ensure the content of this header is not valid
C, albeit being of course valid C++.
Fix unrelated typos in comments.
(joe.ll): Adjust, by removing the call to 'printf'.
* THANKS: Add entry for reporter "Adam Mercer".

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>