Don't include system headers in php_config.h

[cmb69]

Configuration headers (config.h, etc.), as their name implies, are supposed to contain macro definitions to avoid having them to pass on the command line[1]. It seems fine if they include other configuration headers when needed. In any way, they are supposed to be included very early in compilation units which require them.

Doing more than defining simple macros, possibly guarded by defines set during compile time (such as a compiler or platform identification), can lead to subtle issues, which may go unnoticed such as the fallback definitions in the hard-coded (!) fpm_config.h, which may actually be used unconditionally, because the appropriate system headers have not
already been included.

[1] https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Configuration-Headers.html

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So far this is only a partial clean-up of the different configuration files; particularly, almost all of fpm_config.h should be moved elsewhere (likely fpm.h). Before I do this, I'm asking for consensus on the approach outlined above.

Note that this PR is in no way supposed to clean up the module mess we have, nor the include mess, e.g.

php-src/sapi/fpm/fpm/fpm_main.c

Lines 22 to 35 in 07cd468

	#include "php.h"
	#include "php_globals.h"
	#include "php_variables.h"
	#include "php_ini_builder.h"
	#include "zend_modules.h"
	#include "php.h"
	#include "zend_ini_scanner.h"
	#include "zend_globals.h"
	#include "zend_stream.h"
	#include "SAPI.h"
	#include <stdio.h>
	#include "php.h"

Includes php.h three times in not even 15 lines of code.