assert-h: new module, which supports C1X-style static_assert

assert-h: new module, which supports C1X-style static_assert

* lib/assert.in.h, m4/assert_h.m4, modules/assert-h: New files.
* lib/verify.h: Revamp so that this can be copied into assert.h,
while retaining the ability to use it standalone as before.
Rename private identifiers so as not to encroach on the
standard C namespace, since this is now used by assert.h.
(_GL_VERIFY_TYPE): New macro, factoring out differing parts of
the old verify_true.
(_GL_VERIFY_TRUE): New macro, with much of the contents of
the old verify_true.  Use _GL_VERIFY_TYPE.
(_GL_VERIFY): New macro, with much of the contents of the old verify.
(static_assert): New macro, if _GL_STATIC_ASSERT_H
is defined and static_assert is not; _GL_STATIC_ASSERT_H is
defined when this file is copied into the replacement assert.h.
(_Static_assert): New macro, if _GL_STATIC_ASSERT_H is defined
and _Static_assert is not built in.
(verify_true, verify): Define only if _GL_STATIC_ASSERT_H is not
defined, and use the new macros mentioned above.
* doc/posix-headers/assert.texi: Document this.