Port to QNX

Port to QNX

Simplified version of a patch proposed by Elad Lahav in:
https://lists.gnu.org/archive/html/emacs-devel/2017-10/msg00716.html
which is based on a previous patch I proposed in:
https://lists.gnu.org/archive/html/emacs-devel/2017-10/msg00707.html
* configure.ac (opsys, CFLAGS, LIBS_SYSTEM, hybrid_malloc)
(system_alloc, FIRST_PTY_LETTER, CYGWIN_OBJ):
Set appropriately for QNX.
* src/unexelf.c [__QNX__]: Include <sys/elf.h> instead of <elf.h>.
(unexec): Check for sbrk failure, and fall back on old BSS end.