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

(cherry picked from commit 238564b86a506b7ec6e8a3b7b6be7a423cb50337)