Port better to POSIX hosts lacking _setjmp.

Port better to POSIX hosts lacking _setjmp.

* configure.ac (HAVE__SETJMP, HAVE_SIGSETJMP): New symbols.
(_setjmp, _longjmp): Remove.
* src/lisp.h: Include <setjmp.h> here, since we use its symbols here.
All instances of '#include <setjmp.h>' removed, if the
only reason for the instance was because "lisp.h" was included.
(sys_jmp_buf, sys_setjmp, sys_longjmp): New symbols.
Unless otherwise specified, replace all uses of jmp_buf, _setjmp,
and _longjmp with the new symbols.  Emacs already uses _setjmp if
available, so this change affects only POSIXish hosts that have
sigsetjmp but not _setjmp, such as some versions of Solaris and
Unixware.  (Also, POSIX-2008 marks _setjmp as obsolescent.)
* src/image.c (_setjmp, _longjmp) [HAVE_PNG && !HAVE__SETJMP]: New macros.
(png_load_body) [HAVE_PNG]:
(PNG_LONGJMP) [HAVE_PNG && PNG_LIBPNG_VER < 10500]:
(PNG_JMPBUF) [HAVE_PNG && PNG_LIBPNG_VER >= 10500]:
Use _setjmp and _longjmp rather than sys_setjmp and sys_longjmp,
since PNG requires jmp_buf.  This is the only exception to the
general rule that we now use sys_setjmp and sys_longjmp.
This exception is OK since this code does not change the signal
mask or longjmp out of a signal handler.

Fixes: debbugs:12446