lgamma() returns the logarithm of the absolute value of gamma
    of x, or ln(|G(x)|), where G is the gamma function. The sign of
    gamma of x is returned in the external integer variable signgam
    as +1 or -1. The x parameter cannot be 0 or a negative integer.

    gamma() returns the natural log of the gamma function and so is
    functionally equivalent to lgamma(). Because of this, gamma()  is
    marked TO BE WITHDRAWN in the X/Open Portability Guide, Revision
    4 (XPG4).

                           Entry-Point Names

    Generic
    Function   Data Type   OpenVMS                Tru64 UNIX
    Name       Required

    lgamma     S_FLOAT     math$lgamma_s          lgammaf
               T_FLOAT     math$lgamma_t          lgamma
               X_FLOAT     math$lgamma_x          lgammal
               F_FLOAT     math$lgamma_f
               G_FLOAT     math$lgamma_g