Fast, reasonably accurate 10^x. Alternative of detection of libm exp10 at configure
time is not worth the trouble, since it is anyway not POSIX or ISO C,
and currently only the GNU libm has it. Furthermore, GNU libm's variant
is ~ 2x slower, and is ironically not correctly rounded (2 ulp off) to justify all
that slowdown.
Reviewed-by: James Almer <jamrial@gmail.com>
Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>