[libstdc++] Fix PR libstdc++/3988, typo in libmath/stubs.c

[Benjamin Kosnik]

This looks fine, and I'll apply it in a bit. 

 Thanks Peter!

> 2001-08-14  Peter Schmid  <schmid@snake.iap.physik.tu-darmstadt.de>
> 
>         * libstdc++-v3/libmath/stubs.c: Fix PR/3988, replace
>           _GBLICPP_HAVE_LOGL with HAVE_LOGL