based on patch by Dan Gohman, who caught this via compiler warnings.
analysis by Szabolcs Nagy determined that it's a bug, whereby errno
can be set incorrectly for values where the coercion from long double
to double causes rounding. it seems likely that floating point status
flags may be set incorrectly as a result too.

at the same time, clean up use of preprocessor concatenation involving
LDBL_MANT_DIG, which spuriously depends on it being a single unadorned
decimal integer literal, and instead use the equivalent formulation
2/LDBL_EPSILON. an equivalent change on the printf side was made in
commit bff6095d.