rational.hpp::gcd returns a negative value sometimes

[dbenbenn]

boost/rational.hpp provides a gcd function, which is
supposed to be non-negative.  It sometimes returns a
negative value, which can cause problems in other parts
of rational.hpp.  For example, assuming long is a
4-byte type,

boost::gcd<long>(6, -2147483648)

returns -2.  As a result,

boost::rational<long>(-1073741821, 6) + 
boost::rational<long>(-1073741827, 6)

produces an invalid rational number, 1073741824/-3,
instead of the correct answer -1073741824/3.

Here is a small patch to fix the problem.  Note that
this is how boost/math/common_factor_rt.hpp calculates
the greatest common divisor.

[Daryle Walker]

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user_id=551024

Fixed this by depreciating boost::gcd and lcm for the
ones in boost::math (i.e. in "common_factor_rt.hpp").
This forces the requirement of std::numeric_limits
compatibility for your integer type (otherwise you may
get negative numbers during GCD/LCM for signed
types). Your issue is only for a "long" that uses 2's
complement and all its bit combinations, so there's a
preprocessor-level check for that in the test code.