[Bug 233481] flex/lex: __dead2 define in flex.skl precludes cross compiles for embedded systems

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=233481

            Bug ID: 233481
           Summary: flex/lex: __dead2 define in flex.skl precludes cross
                    compiles for embedded systems
           Product: Base System
           Version: 11.2-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: bugs at FreeBSD.org
          Reporter: leres at freebsd.org

We're unable to compile a lex generated module for the openvehicles.com project
with the devel/xtensa-esp32-elf toolchain. The problem is the flex/lex
generated code unconditionally defines __dead2 when !defined(__FreeBSD__):

    In file included from
/home/leres/esp/openvehicles-xtensa-esp32-elf/components/newlib/include/stdio.h:35:0,
                     from
/home/leres/src/Open-Vehicle-Monitoring-System-3/vehicle/OVMS.V3/components/dbc/yacclex/dbc_tokeniser.c:30:
   
/home/leres/esp/openvehicles-xtensa-esp32-elf/components/newlib/include/sys/cdefs.h:246:0:
warning: "__dead2" redefined
     #define __dead2  __attribute__((__noreturn__))
     ^
   
/home/leres/src/Open-Vehicle-Monitoring-System-3/vehicle/OVMS.V3/components/dbc/yacclex/dbc_tokeniser.c:26:0:
note: this is the location of the previous definition
     #define __dead2

The problem is the generated code unconditionally defines __dead2:

    /* First, we deal with  platform-specific or compiler-specific issues. */

    #if defined(__FreeBSD__)
    #ifndef __STDC_LIMIT_MACROS
    #define __STDC_LIMIT_MACROS
    #endif
    #include <sys/cdefs.h>
    #include <stdint.h>
    #else
    #define __dead2
    #endif

And then later includes stdio.h:

    /* begin standard C headers. */
    #include <stdio.h>
    #include <string.h>
    #include <errno.h>
    #include <stdlib.h>

    /* end standard C headers. */

But stdio.h causes the xtensa-esp32-elf version sys/cdefs.h to be included
which defines __dead2 for the second time:

    #if __GNUC_PREREQ__(2, 7)
    #define __dead2         __attribute__((__noreturn__))
    [...]

The only workaround is to do hacky post-processing on the generated code (or
perhaps use the textproc/flex port which does not patch the standard flex.skl
file).

The attached patch moves the __dead2 define after the "standard C headers" are
included and only attempts to define __dead2 if it is not already defined.

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