Define _USE_32BIT_TIME_T for VC6 and VC7

Define _USE_32BIT_TIME_T for VC6 and VC7

In VS 2005 (VC++ 8.0) Microsoft changes time_t from 32-bit to
64-bit, even in 32-bit mode.  It also provides the _USE_32BIT_TIME_T
preprocessor option to revert back to the old functionality for
backward compatibility.  We define this symbol here for older 32-bit
compilers only (which aren't using it at all) for the sole purpose
of getting it into $Config{ccflags}.  That way if someone builds
Perl itself with e.g. VC6 but later installs an XS module using VC8
the time_t types will still be compatible.