Copy pc_from_ucontext.m4 from gperftools.

https://code.google.com/p/gperftools/
Fix pc_from_ucontext.m4.

* Add rip as one of pc_fields.
* Add eip as one of pc_fields.
* Include sys/types.h for cygwin/signal.h.
* Add missing typedef for ucontext_t.
Use AC_PC_FROM_UCONTEXT.

* Update AC_CHECK_HEADERS for execinfo.h, sys/ptrace.h, sys/sysctl.h
  and sys/prctl.h.
* Define NO_STACK_TRACE via AC_PC_FROM_UCONTEXT.
* Include headers via HAVE_*_H.
* Use PC_FROM_UCONTEXT instead of ARCH_IP_REG.
* Add #ifdef for execinfo.h specific codes.
Merge pull request #25 from hiyuh/ucontext

Make NO_STACK_TRACE handling better.