* configure.in: Add configuration test for long double equals dbl
and set flag _LDBL_EQ_DBL if true.
* configure: Regenerated.
* newlib.hin: Add _LDBL_EQ_DBL flag.
* libc/include/math.h: Use _LDBL_EQ_DBL flag instead of
_HAVE_LDBL_MATH.
* libc/include/stdlib.h: Use _LDBL_EQ_DBL flag instead of
_HAVE_LDBL_STDLIB.
* libc/common/local.h: Remove _LDBL_EQ_DBL flag setting.
* libc/stdlib/local.h: Ditto.