build-sys: improve detection of the "isnan" function in uClibc

build-sys: improve detection of the "isnan" function in uClibc

Since commit beceb14b450ded6560ed743634a5e80604a8edf3, MATH_LIBS is set
to "-lm" when the isnan function is detected. In uClibc, however, isnan
is a macro that calls __isnan, __isnanf, or __isnanl, depending on the
size of the argument (double, float or long double).

Fixes:
  http://autobuild.buildroot.net/results/2c2/2c29a78ed81ca844a87dcd076ab3e14ea080296d/
  http://autobuild.buildroot.net/results/404/404b10f359b2ae8a7216729fa1bab37fed2d3d4c/

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>