GCC 14 and 15 default to -std=gnu23, where 'bool' is a keyword.  This
causes two compile errors in IvorySQL-specific code:

1. src/interfaces/libpq/libpq-fe.h defines 'typedef unsigned char bool'.
   The existing guard '#if !(defined(true) && defined(false))' is not
   sufficient under C23, because true/false may not be defined as macros
   even though bool is a built-in keyword.  Add a check for __STDC_VERSION__
   >= 202311L (and __cplusplus) to skip the typedef, mirroring the pattern
   used in src/include/c.h.

2. contrib/ivorysql_ora/.../ora_xml_functions.c: ivy_xmlisvalid() returns
   Datum but the !USE_LIBXML fallback did 'return NULL'.  With GCC 14's
   -Wint-conversion promoted to error, this fails.  Replace with 'return 0'
   to match the upstream PostgreSQL xml.c pattern.

Fixes IvorySQL#1351.

Replace the custom bool typedef with <stdbool.h>, as requested in the review of PR IvorySQL#1385.

Assisted-by: OpenAI:gpt-5

…23-bool

Fix build failure with GCC 14+ (C23 bool keyword and -Wint-conversion)