[Bug-gsl] Issue with log1p

[Brian Gladman]

On platforms that don't have a native implementation of log1p, GSL 
substitutes gsl_log1p.

But in many files where log1p is used, the gsl_sys.h header (where gsl_log1p 
is declared) is not included so the compiler doesn't pick up the declaration 
of gsl_log1p and hence wrongly concludes that it returns an int.

So the header gsl_sys.h needs to be included in all files where log_1p is 
used.

This also applies to other functions that seek to substitute for missing 
'standard' functions.

I picked this up on the Microsoft compiler but it also impacts on the Intel 
compiler on Windows.

  best regards,

    Brian Gladman