Returns the natural logarithm of the absolute value of the Gamma function.
Remember to link the math library:
valac -X -lm ...
The sign of the Gamma function is returned in the external integer Posix.signgam. It is 1 when the Gamma function is positive or zero, -1 when it is negative.
Since using a constant location GLib.signgam is not thread-safe, the functions GLib.Math.lgamma_r etc. have been introduced; they return this sign via an output argument.
A numeric value.