Revert the PL/Perl-specific change in
9a95a77d.  We must not prevent Perl from
using stdbool.h when it has been built to do so, even if it uses an
incompatible size.  Otherwise, we would be imposing our bool on Perl,
which will lead to crashes because of the size mismatch.

Instead, we undef bool after including the Perl headers, as we did
previously, but now only if we are not using stdbool.h ourselves.
Record that choice in c.h as USE_STDBOOL.  This will also make it easier
to apply that coding pattern elsewhere if necessary.