Remove obsolete and unused configure macros - #159513

Remove obsolete platform workarounds and configuration checks whose results are no longer used:

• Remaining IRIX workarounds: Remove IRIX-specific declarations, header handling, and macros from socket, select, termios, curses, posix, and integer-conversion code. IRIX support was removed in Python 3.7.
• Checks left behind by removed modules: Remove checks for shadow.h, getspent(), getspnam(), linux/soundcard.h, sys/soundcard.h, and sys/audioio.h. Their consumers were the removed spwd, ossaudiodev, and sunaudiodev modules.
• Checks left behind by bundled libmpdec: Remove the glibc _FORTIFY_SOURCE/memmove bug check, GCC ipa-pure-const bug check, and x64 inline-assembler check. Their consumers were removed with bundled libmpdec in 9eb3b14. Also remove the -O2 availability check used only by the glibc bug check.
• Unused function checks: Remove checks for ftime(), getwd(), tempnam(), tmpnam(), tmpnam_r(), sigrelse(), fseek64(), and ftell64(). Neither their configuration macros nor their cached results have remaining consumers.
• Pre-ANSI compiler checks: Remove checks for working const, signed char, and function prototypes, along with their compatibility macros. These features are guaranteed by CPython’s C11 requirement.
• Obsolete MSVC version conditions: Remove conditions for compiler versions older than the supported minimum, retaining the current implementations and non-MSVC alternatives. This covers Windows configuration, socket headers, secure CRT handling, and invalid-parameter handlers.
• Legacy BSD workarounds: Remove _XOPEN_SOURCE exceptions for OpenBSD 2.x–4.6 and NetBSD 1.5–1.6-era releases, and the pre-December-1997 NetBSD dynamic-loader workaround.
• Regenerate configure and pyconfig.h.in with Autoconf 2.72, and update the C API macro allowlist and platform headers accordingly.