define tgamma that'd compute natural logarithm of gamma function,
and not gamma function itself, therefore completely different values
e.g. parser 4! yields 3.178054 which is obviously wrong
though on vax this patch used to be right. :-)
tested by, and ok sthen@
this is totally experimental and not working yet !
it's in so that experimental people can look at it and play with it.
do not even think of building it if you don't know what you're doing.
