fixed check for sinl in acinclude.m4

[Benjamin Kosnik]

>   I though about that, but didnt figure it'd hurt to try to catch any future
> additions.  

Let's add them when they get added to the compiler, ok? 

>Anyway, it does appear neccessary to link against libm for
> the builtins

No. That's the whole point. Try just doing a simple case for 
__builtin_sinf: the x86 compiler converts this to a call to sinf.

Thus, you can't link against libm (which has a def for sinf) if you want 
to detect what calls actually are __builtin....

Anyway. 

This code is now working correctly..... it might help if you can compile 
on different platforms than x86. It becomes amazingly clear what's up 
when you do this....

;)

-benjamin