Remove PHP atomic includes and PHP_DEFINE M4 macro (#13372)

PHP_DEFINE was introduced with the PHP 5 build system
9d9d39a0de and then refactored via
350de12bc2.

This was once used to put defined constants into a single file to have
more fine-graned dependencies (atomic includes). Since no known PHP
extension is using this and it makes very little sense to use this, this
M4 macro can be removed in favor of the Autoconf native way using
AC_DEFINE and the usual included files php_config.h and config.h.

- Generated unused include directory removed
- Remove include dir from DEFS
- Remove also include dir from PDO checks

docs/unix-build-system.md

@@ -7,8 +7,6 @@
   especially noticeable on slower systems
 * slow recursive make replaced with one global Makefile
 * eases integration of proper dependencies
-* adds PHP_DEFINE(what[, value]) which creates a single include-file per what.
-  This will allow more fine-grained dependencies.
 * abandoning the "one library per directory" concept
 * improved integration of the CLI
 * several new targets: