Returns the gamma function of its argument.
    This function is OpenVMS Alpha and Integrity servers only.
    Format
      #include  <math.h>
      double tgamma  (double x);
      float tgammaf  (float x);
      long double tgammal  (long double x);
1 – Argument
 x
    A real value.
2 – Description
The tgamma functions compute the gamma function of x.
3 – Return Values
    n                  Upon success, the gamma function of x.
    -1                 If x is negative. errno is set to EDOM.
    HUGE_VAL           Overflow occurred, or x is 0. errno is set to
                       ERANGE.
    NaN                If x is NaN or -Inf. errno is set to EDOM.
    x                  If x is +Inf.