Enable Large File Support (LFS) on 32-bit systems by default

In line with the structure of the existing code, this is done by
adding '#define _FILE_OFFSET_BITS 64' near the top of civetweb.c,
before the various system #includes.

This might not work well if the code is later split up into more
translation units; in that case, it would be better to move the LFS
macros into a separate header file that is included as appropriate.
It may also be more portable to enable LFS via the makefile instead,
for example, by adding something like:

  ifdef WITH_LFS
    CFLAGS += $(shell getconf LFS_CFLAGS)
    LDFLAGS += $(shell getconf LFS_LDFLAGS)
  endif