Overhaul CMake LFS support (#4122)

Externally visible:
* The HDF_ENABLE_LARGE_FILE option (advanced) has been removed
* We no longer run a test program to determine if LFS works, which
  will help with cross-compiling
* On Linux we now unilaterally set -D_LARGEFILE_SOURCE and
  -D_FILE_OFFSET_BITS=64, regardless of 32/64 bit system. CMake
  doesn't offer a nice equivalent to AC_SYS_LARGEFILE and since
  those options do nothing on 64-bit systems, this seems safe and
  covers all our bases. We don't set -D_LARGEFILE64_SOURCE since
  we don't use any of the POSIX 64-bit specific API calls like
  ftello64, as noted above.
* We didn't test for LFS support on non-Linux platforms. We've added
  comments for how LFS should probably be supported on AIX and Solaris,
  which seem to be alive, though uncommon. PRs would be appreciated if
  anyone wishes to test this.

Internal:
* Drops off64_t size checks since this is unused (as in Autotools)
* Remove HDF_EXTRA_FLAGS, which is now unused
* Remove hack around deprecated LINUX_LFS

Fixes #2395

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