Remove use of obsolescent Autoconf macros

Remove the use of the following macros, which are obsolescent according
to the Autoconf documentation:

- AC_C_CONST
- AC_C_STRINGIZE
- AC_C_VOLATILE
- AC_FUNC_MEMCMP

configure.in

@@ -1114,12 +1114,9 @@ fi
 
 m4_defun([AC_PROG_CC_STDC], []) dnl We don't want that.
 AC_C_BIGENDIAN
-AC_C_CONST
 PGAC_C_INLINE
-AC_C_STRINGIZE
 AC_C_FLEXIBLE_ARRAY_MEMBER
 PGAC_C_SIGNED
-AC_C_VOLATILE
 PGAC_C_FUNCNAME_SUPPORT
 PGAC_C_STATIC_ASSERT
 PGAC_C_TYPES_COMPATIBLE
@@ -1749,9 +1746,6 @@ operating system;  use --disable-thread-safety to disable thread safety.])
 fi
 fi
 
-# SunOS doesn't handle negative byte comparisons properly with +/- return
-AC_FUNC_MEMCMP
-
 
 # Select semaphore implementation type.
 if test "$PORTNAME" != "win32"; then