In Perl_feature_is_enabled() use cBOOL to convert the pointer to a "bool".

In Perl_feature_is_enabled() use cBOOL to convert the pointer to a "bool".

On some platforms which don't have a (real) bool type, bool is actually a
char, and hence (sadly) it's correct for the compiler to truncate when
assigning to it, instead of what the programmer thought was going to happen
(testing zero or not). In Perl_feature_is_enabled(), the expression is a
pointer, so if it converts to an integer with the bottom 8 bits zero, then
on these platforms it would truncate to "false". Not what was expected.