e838a98 depends: re-enable using -flto when building expat (fanquake)
3044525 depends: expat 2.4.8 (fanquake)

Pull request description:

  Currently, when building the expat package in depends, using `-flto` (`LTO=1`), the configure check can fail, because it cannot determine the system endianess:
  ```bash
  configure:18718: result: unknown
  configure:18733: error: unknown endianness
   presetting ac_cv_c_bigendian=no (or yes) will help
  ```

  Fix that by defining `_DEFAULT_SOURCE`, which in turn defines `__USE_MISC` (`features.h`):
  ```c
  #if defined _DEFAULT_SOURCE
  # define __USE_MISC1
  #endif
  ```
  which exposes additional definitions in `endian.h`:
  ```c
  #include <features.h>

  /* Get the definitions of __*_ENDIAN, __BYTE_ORDER, and __FLOAT_WORD_ORDER.  */
  #include <bits/endian.h>

  #ifdef __USE_MISC
  # define LITTLE_ENDIAN__LITTLE_ENDIAN
  # define BIG_ENDIAN__BIG_ENDIAN
  # define PDP_ENDIAN__PDP_ENDIAN
  # define BYTE_ORDER__BYTE_ORDER
  #endif
  ```
  and gives us a working configure.

  You could test building this change with Guix + LTO with [this branch](https://github.com/fanquake/bitcoin/tree/lto_in_guix). Note that that build may fail for other reasons (on x86_64), unrelated to this change.

  Some related upstream discussion:
  https://bugs.gentoo.org/757681
  https://forums.gentoo.org/viewtopic-t-1013786.html

ACKs for top commit:
  hebasto:
    re-ACK e838a98, only [suggested](bitcoin#25697 (comment)) changes since my recent [review](bitcoin#25697 (review)).
  jarolrod:
    code review ACK e838a98

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