fixed check for sinl in acinclude.m4

[Benjamin Kosnik]

Just because the compiler can parse __builtinfoo, doesn't mean that it 
does something optimal, or even anything more useful than bailing to the 
appropriate libm function with it. 

If builtins are going to use libm functions, there is no sense in using the 
builtins, and the <math.h> and libm stuff should be used directly. 

Where "no sense" is defined in a practical way, meaning performance increase 
over increased configuration complexity.

later
benjamin