Fix compile errors with certain CFLAGS.

Fix compile errors with certain CFLAGS.

https://github.com/pgpool/pgpool2/issues/42 reported that with CFLAGS
-flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing
gcc emits errors. Some of them are mistakes when their sources were
brought in from PostgreSQL.  This commit fixes them. Note that I was
not able to suppress some errors at least with my gcc (9.4.0). This
may be because gcc bug (false positives) or just a bug with the old
gcc, I don't know at this point. Maybe someday revisit this.

Discussion:
[pgpool-hackers: 4442] Fixing GitHub issue 42
https://www.pgpool.net/pipermail/pgpool-hackers/2024-March/004443.html

../src/include/query_cache/pool_memqcache.h:251:20: warning: type of 'pool_fetch_from_memory_cache' does not match original declaration [-Wlto-type-mismatch]
  251 | extern POOL_STATUS pool_fetch_from_memory_cache(POOL_CONNECTION * frontend,
      |                    ^
query_cache/pool_memqcache.c:731:1: note: 'pool_fetch_from_memory_cache' was previously declared here
  731 | pool_fetch_from_memory_cache(POOL_CONNECTION * frontend,
      | ^
query_cache/pool_memqcache.c:731:1: note: code may be misoptimized unless '-fno-strict-aliasing' is used
../src/include/utils/palloc.h:64:22: warning: type of 'CurrentMemoryContext' does not match original declaration [-Wlto-type-mismatch]
   64 | extern MemoryContext CurrentMemoryContext;
      |                      ^
../../src/utils/mmgr/mcxt.c:40:15: note: 'CurrentMemoryContext' was previously declared here
../../src/utils/mmgr/mcxt.c:40:15: note: code may be misoptimized unless '-fno-strict-aliasing' is used
../src/include/utils/memutils.h:55:22: warning: type of 'TopMemoryContext' does not match original declaration [-Wlto-type-mismatch]
   55 | extern MemoryContext TopMemoryContext;
      |                      ^
../../src/utils/mmgr/mcxt.c:46:15: note: 'TopMemoryContext' was previously declared here
../../src/utils/mmgr/mcxt.c:46:15: note: code may be misoptimized unless '-fno-strict-aliasing' is used
../src/include/pool_config.h:646:22: warning: type of 'pool_config' does not match original declaration [-Wlto-type-mismatch]
  646 | extern POOL_CONFIG * pool_config;
      |                      ^
config/pool_config.l:46:14: note: 'pool_config' was previously declared here
   46 | POOL_CONFIG *pool_config = &g_pool_config; /* for legacy reason pointer to the above struct */
      |              ^
config/pool_config.l:46:14: note: code may be misoptimized unless '-fno-strict-aliasing' is used