Move LFS64 detection from build system to C preprocessor

Move LFS64 detection from build system to C preprocessor

zbuild.h now defines _LARGEFILE64_SOURCE before any system header, so
glibc exposes off64_t and lseek64 where available. The _LFS64_LARGEFILE
check already in zconf.h handles per-platform gating. Drops the
redundant -D__USE_LARGEFILE64 (internal glibc macro, set automatically)
and the unused __off64_t probe that only wrote to HAVE___OFF64_T.

The configure script's _off64_t probe was log-only and the off64_t=yes
path assumed fseeko exists without testing; both are removed in favor
of the standalone fseeko check.