Description:
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Compilation of phpdbg on Solaris 10 Sparc fails:

.../sapi/phpdbg/phpdbg_utils.c: In function 'phpdbg_set_async_io':
.../sapi/phpdbg/phpdbg_utils.c:353:39: error: 'FASYNC' undeclared (first use in this function)
  fcntl(STDIN_FILENO, F_SETFL, flags | FASYNC);

On Solaris FASYNC is defined in sys/file.h:

...
/* open-only modes */

#define FCREAT          0x0100
#define FTRUNC          0x0200
#define FEXCL           0x0400
#define FNOCTTY         0x0800
#define FXATTR          0x4000  /* open as extended attribute */

#define FASYNC          0x1000  /* asyncio in progress pseudo flag */
#define FNODSYNC        0x10000 /* fsync pseudo flag */

#define FNOFOLLOW       0x20000 /* don't follow symlinks */
#define FNOLINKS        0x40000 /* don't allow multiple hard links */
...

As a workaround I added "#include <sys/file.h>" to phpdbg.h.

I think you need to use auto foo to detect existence of FASYNC and if not present check again with using sys/file.h to decide, whether you need to include it. This is unfortunately outside of POSIX et. al., so platform specific.