lgamma returns NaN when given a negative integer but should return Inf

[bgoodri]

Summary:

lgamma(-1) returns NaN, which is documented and what Boost does but stupid

Description:

The limit of the Gamma function as its argument approaches a non-negative integer is + / - infinity depending on whether you approach from the right or the left. The C++11 version of lgamma returns positive infinity, as does R. We should switch to the C++11 version of all these functions in cmath.

Reproducible Steps:

Evaluate lgamma at a non-positive integer

Current Output:

NaN

Expected Output:

inf

Additional Information:

Current Version:

v2.15.0

[andrjohns]

Closed via #1080