Pull up following revision(s) (requested by riastradh in ticket #506):

lib/libc/gdtoa/Makefile.inc: revision 1.11
lib/libc/gdtoa/gdtoa_fltrnds.h: revision 1.2
lib/libc/gdtoa/gdtoaimp.h: revision 1.15
lib/libc/gdtoa/gdtoaimp.h: revision 1.17

Honour the floating-point rounding mode in floating-point formatting.

C99, Sec. 7.19.6.1 `The fprintf function', paragraph 13, p. 281:
  (Recommended practice)
  For e, E, f, F, g, and G conversions, if the number of significant
  decimal digits is at most DECIMAL_DIG, then the result should be
  correctly rounded.  If the number of significant decimal digits is
  more than DECIMAL_DIG but the source value is exactly
  representable with DECIMAL_DIG digits, then the result should be
  an exact representation with trailing zeros.  Otherwise, the
  source value is bounded by two adjacent decimal strings L < U,
  both having DECIMAL_DIG significant idgits; the value of the
  resultant decimal string D should satisfy L <= D <= U, _with the
  extra stipulation that the error should have a correct sign for
  the current rounding direction_.  [emphasis added]

The gdtoa code base already supports respecting the floating-point
rounding mode, as long as we compile it with Honor_FLT_ROUNDS
defined.  However, for this to work, fegetround must be available in
libc, which it is not currently -- the fenv logic is in libm.

Fortunately, we don't have to move all of fenv from libm to libc --
programs that do not link against libm don't have fesetround, so the
rounding mode is always the default (barring asm shenanigans that
bypass the API -- tough).  So use a weak reference to fegetround; by
default, assume FE_TONEAREST if it is not defined.

Mark the libc fegetround weak reference unused.
Not all .c files that include gdtoaimp.h use it, which makes clang
unhappy.

(martin)