The libc++experimental library was added recently, but one part of it
(std::experimental::filesystem) contains a number of flawed static
assertion tests, which assume time_t is always 64 bit on LP64
systems, and 32 bit otherwise.
Change the preprocessor guards to test for TIME_T_64BIT instead, which
we can then define appropriately for the platform in the Makefile.
Not that for -CURRENT, as of recently, the only arch left with 32 bit
time_t is i386. If this ever gets MFC'd, that would also include 32 bit
PowerPC, IIRC.