Don't check long double avaiability for BOOST_CSTDFLOAT_xx.

  For example IA-32, those macros suggest long double as 80-bit floating
  point type, however some math runtime implementations are broken for
  long doublle even if supported by compiler (e.g. FreeBSD 9, ...).

Signed-off-by: Kohei Takahashi <flast@flast.jp>