tests: avoid failure due to libtool quirks in C++ demo test

tests: avoid failure due to libtool quirks in C++ demo test

Revealed by failures on NetBSD 5.1.

* t/cxx-lt-demo.sh: In "make distcheck" invocation, don't define 'CC'
to "false", as that value would be exported and passed to the child
./configure invocation, and some configure checks generated by libtool
autoconf macros can still require a C preprocessor even for packages
using only C++, and bail out if it's not found.  The problem was not
apparent on Solaris and GNU/Linux because those systems have a
'/lib/cpp' program, and configure detected and used that as a fallback
C preprocessor.

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