math/givaro: only declare std::bool_constant if it does not exist yet
While doing test builds for bug 284159 (math/linbox), I ran into an
issue compiling against one of the math/givaro headers, with clang and
libc++ 19:
/usr/local/include/givaro/random-integer.h:40:14: error: reference to 'bool_constant' is ambiguous
40 | std::bool_constant<_Exact_Size>::type _Exact_Size_t;
| ^
/usr/include/c++/v1/__type_traits/integral_constant.h:42:1: note: candidate found by name lookup is 'std::__1::bool_constant'
42 | using bool_constant = integral_constant<bool, __b>;
| ^
/usr/local/include/givaro/random-integer.h:16:5: note: candidate found by name lookup is 'std::bool_constant'
16 | using bool_constant = integral_constant<bool, B>;
| ^The problem is that random-integer.h attempts to declare
std::bool_constant, even though libc++ headers already have this
template.
Conditionalize it on __cpp_lib_bool_constant [1], so it is only declared
when necessary (i.e. if somebody compiles against and older C++
library).
[1] https://en.cppreference.com/w/cpp/feature_test#cpp_lib_bool_constant
PR: 284167
Approved by: pi (maintainer)
MFH: 2025Q1