Add gnulib support for pthread_sigmask.

Add gnulib support for pthread_sigmask.

* Makefile.in (GNULIB_MODULES): Add pthread_sigmask.
(GNULIB_TOOL_FLAGS): Avoid sigprocmask.  Emacs does its own
implementation of 'sigprocmask' on Windows, and it assumes
'sigprocmask' on non-Windows hosts, so it doesn't need the
sigprocmask module.
* lib/signal.in.h, m4/pthread_sigmask.m4, m4/signal_h.m4:
New files, automatically imported from gnulib.
* lib/gnulib.mk, m4/gl-comp.m4: Automatically-imported update
due to the above changes.
* .bzrignore: Add lib/signal.h.