configure: Do not add -D_FILE_OFFSET_BITS to CFLAGS AC_SYS_LARGEFILE macro is in use and this will add a definition for _FILE_OFFSET_BITS in generated config.h which is already included as first include file in fatresize.c hence its not required to be added via CFLAGS, this also fixes a case when -D_FILE_OFFSET_BITS=64 is passed via CC from environment, where the autoconf macros set ac_cv_sys_file_offset_bits=no and that means we will have -D_FILE_OFFSET_BITS=no added to CFLAGS which messes up builds.
