Merge bitcoin/bitcoin#29484: serialization: replace char-is-int8_t autoconf detection with c++20 concept

ad7584d8b6 serialization: replace char-is-int8_t autoconf detection with c++20 concept (Cory Fields)

Pull request description:

  Doesn't depend on #29263, but it's really only relevant after that one's merged.

  This removes the only remaining autoconf macro in our serialization code (after #29263), so it can now be used trivially and safely out-of-tree.

  ~Our code does not currently contain any concepts, but couldn't find any discussion or docs about avoiding them. I guess we'll see if this blows up our c-i.~
  Edit: Ignore this. ajtowns pointed out that we're already using a few concepts.

  This was introduced in #13580. Please check my logic on this as I'm unable to test on a SmartOS system. Even better would be a confirmation from someone who can build there.

ACKs for top commit:
  Empact:
    Code review ACK ad7584d8b6

Tree-SHA512: 1faf65c900700efb1cf3092c607a2230321b393cb2f029fbfb94bc8e50df1dabd7a9e4b91e3b34f0d2f3471aaf18ee7e56d91869db5c5f4bae84da95443e1120

configure.ac

@@ -1171,14 +1171,6 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/types.h>
  [ AC_MSG_RESULT([no])]
 )
 
-AC_MSG_CHECKING([for if type char equals int8_t])
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <stdint.h>
-  #include <type_traits>]],
- [[ static_assert(std::is_same<int8_t, char>::value, ""); ]])],
- [ AC_MSG_RESULT([yes]); AC_DEFINE([CHAR_EQUALS_INT8], [1], [Define this symbol if type char equals int8_t]) ],
- [ AC_MSG_RESULT([no])]
-)
-
 AC_MSG_CHECKING([for fdatasync])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <unistd.h>]],
  [[ fdatasync(0); ]])],