test-stdbool: skip test that fails with Solaris CC

test-stdbool: skip test that fails with Solaris CC

Solaris 9 CC has a bug; it allows (int)0.0 and (bool)0 in constant
expressions, but not (bool)0.0.  Our replacement stdbool cannot
provide that guarantee of C99 for this particular compiler, but
since use of that feature causes compilation failure rather than
silent mis-compilation, we just skip the test.

* tests/test-stdbool.c (f): Skip test that causes compilation
error under buggy C++ compiler.
* lib/stdbool.in.h: Document the limitation.
* doc/posix-headers/stdbool.texi (stdbool.h): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>