gengtype: Include system.h earlier in gengtype-lex.cc [PR121386]

gengtype: Include system.h earlier in gengtype-lex.cc [PR121386]

OpenBSD headers apparently instead of just
 #define SIZE_MAX something
do
 #ifndef SIZE_MAX
 #define SIZE_MAX something
 #endif
This causes problem with gengtype-lex.cc, where the flex generated
code has
 #ifndef SIZE_MAX
 #define SIZE_MAX (~(size_t)0)
 #endif
and system.h is included only after that and since my changes for
host size_t *printf printing SIZE_MAX is used in preprocessor
expressions,
 #if SIZE_MAX <= UINT_MAX
etc.
In the preprocessor, identifiers are replaced with 0 and so
it is (~(0)0) <= 0xffffffffU
or so and thus invalid.

Now, normally we want to include system.h early, ideally immediately
after config.h or bconfig.h, but in gengtype-lex.cc case we have
 #ifdef HOST_GENERATOR_FILE
 #include "config.h"
 #define GENERATOR_FILE 1
 #else
 #include "bconfig.h"
 #endif

 // flex generated start of file, including
 #include <stdio.h>
 #include <string.h>
 #include <errno.h>
 #include <stdlib.h>
 #include <inttypes.h>
 #ifndef SIZE_MAX
 #define SIZE_MAX (~(size_t)0)
 #endif

 // start of gengtype-lex.l %{} section
 #ifdef HOST_GENERATOR_FILE
 #include "config.h"
 #define GENERATOR_FILE 1
 #else
 #include "bconfig.h"
 #endif
 #include "system.h"

 #define malloc xmalloc
 #define realloc xrealloc

 #include "gengtype.h"

As I'm not sure what flex we require for building gcc (%top{} which COBOL FE
*.l uses is only in flex from 2003-04-01), the patch keeps using the %top{}
done by hand in Makefile.in, but includes system.h in the top part, with
FLEX_SCANNER temporarily defined (I'm undefining it afterwards because
flex generated code defines it again and I don't want to guarantee it is
defined to the same value) so that malloc/realloc poisoning doesn't happen
and #define malloc xmalloc and realloc xrealloc are done in system.h.
Note, system.h already includes all the 5 headers flex generated code
includes.

2025-08-06  Jakub Jelinek  <jakub@redhat.com>

	PR bootstrap/121386
	* Makefile.in (gengtype-lex.cc): Append #define FLEX_SCANNER,
	#include "system.h" and #undef FLEX_SCANNER to the prepended lines.
	* gengtype-lex.l: Remove inclusion of config.h or bconfig.h, system.h
	and definition of malloc/realloc from %{} section.