Macro redefinition warning after aeb07c55fab5c17a600b77ffcdc3b71425d6a8e7

When compiling, I get the following warnings:

	[338/2397] Compiling C object src/timezone/zic.p/zic.c.o
	In file included from ../src/timezone/zic.c:22:
	../src/timezone/private.h:187:9: warning: ‘unreachable’ redefined
	  187 | #define unreachable() pg_unreachable()
	      |         ^~~~~~~~~~~
	In file included from ../src/include/c.h:68,
	                 from ../src/include/postgres_fe.h:28,
	                 from ../src/timezone/zic.c:11:
	/nix/store/hkldzpgigap7kkqzdr5j4qqyy5ac4l6x-gcc-16.1.0/lib/gcc/x86_64-unknown-linux-gnu/16.1.0/include/stddef.h:468:9: note: this is the location of the previous definition
	  468 | #define unreachable() (__builtin_unreachable ())
	      |         ^~~~~~~~~~~
	[1218/2397] Compiling C object src/backend/postgres_lib.a.p/.._timezone_strftime.c.o
	In file included from ../src/timezone/strftime.c:45:
	../src/timezone/private.h:187:9: warning: ‘unreachable’ redefined
	  187 | #define unreachable() pg_unreachable()
	      |         ^~~~~~~~~~~
	In file included from ../src/include/c.h:68,
	                 from ../src/include/postgres.h:48,
	                 from ../src/timezone/strftime.c:41:
	/nix/store/hkldzpgigap7kkqzdr5j4qqyy5ac4l6x-gcc-16.1.0/lib/gcc/x86_64-unknown-linux-gnu/16.1.0/include/stddef.h:468:9: note: this is the location of the previous definition
	  468 | #define unreachable() (__builtin_unreachable ())
	      |         ^~~~~~~~~~~
	[1225/2397] Compiling C object src/backend/postgres_lib.a.p/.._timezone_localtime.c.o
	In file included from ../src/timezone/localtime.c:24:
	../src/timezone/private.h:187:9: warning: ‘unreachable’ redefined
	  187 | #define unreachable() pg_unreachable()
	      |         ^~~~~~~~~~~
	In file included from ../src/include/c.h:68,
	                 from ../src/timezone/localtime.c:17:
	/nix/store/hkldzpgigap7kkqzdr5j4qqyy5ac4l6x-gcc-16.1.0/lib/gcc/x86_64-unknown-linux-gnu/16.1.0/include/stddef.h:468:9: note: this is the location of the previous definition
	  468 | #define unreachable() (__builtin_unreachable ())
	      |         ^~~~~~~~~~~
	[1310/2397] Compiling C object src/bin/initdb/initdb.p/.._.._timezone_localtime.c.o
	In file included from ../src/timezone/localtime.c:24:
	../src/timezone/private.h:187:9: warning: ‘unreachable’ redefined
	  187 | #define unreachable() pg_unreachable()
	      |         ^~~~~~~~~~~
	In file included from ../src/include/c.h:68,
	                 from ../src/timezone/localtime.c:17:
	/nix/store/hkldzpgigap7kkqzdr5j4qqyy5ac4l6x-gcc-16.1.0/lib/gcc/x86_64-unknown-linux-gnu/16.1.0/include/stddef.h:468:9: note: this is the location of the previous definition
	  468 | #define unreachable() (__builtin_unreachable ())
	      |         ^~~~~~~~~~~

The upstream code for private.h's definition of unreachable() looks 
quite a bit different than ours[0]. Seems like maybe our defintion was 
over simplified. I don't actually know how the code is pulled in.

Attached it a potential solution based on how we protect the 
static_assert definition in the same file.

[0]: https://github.com/eggert/tz/blob/e14ee1cb61ca948b45f928ad187eaecc5a92d27e/private.h#L1055-L1069

-- 
Tristan Partin
PostgreSQL Contributors Team
AWS (https://aws.amazon.com)