support: Handle clang support/dtotimespec.c on dtotimespec

support: Handle clang support/dtotimespec.c on dtotimespec

clang issues:

dtotimespec.c:31:25: error: implicit conversion from 'time_t' (aka
'long') to 'double' changes value from 9223372036854775807 to
9223372036854775808 [-Werror,-Wimplicit-const-int-float-conversion]
  else if (sec >= 1.0 + TYPE_MAXIMUM (time_t))
                      ~ ^~~~~~~~~~~~~~~~~~~~~
../include/intprops.h:57:4: note: expanded from macro 'TYPE_MAXIMUM'
  ((t) (! TYPE_SIGNED (t)
\
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So explicit cast it to double.

Reviewed-by: Sam James <sam@gentoo.org>