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.

modules/assert-h

@@ -0,0 +1,47 @@
+Description:
+An <assert.h> that conforms to C1X.
+
+Files:
+lib/assert.in.h
+lib/verify.h
+m4/assert_h.m4
+
+Depends-on:
+include_next
+
+configure.ac:
+gl_ASSERT_H
+
+Makefile.am:
+BUILT_SOURCES += $(ASSERT_H)
+
+# We need the following in order to create <assert.h> when the system
+# doesn't have one that works with the given compiler.
+if GL_GENERATE_ASSERT_H
+assert.h: assert.in.h verify.h $(top_builddir)/config.status
+	$(AM_V_GEN)rm -f $@-t $@ && \
+	{ echo '/* DO NOT EDIT! GENERATED AUTOMATICALLY! */' && \
+	  sed -e 's|@''INCLUDE_NEXT''@|$(INCLUDE_NEXT)|g' \
+	      -e 's|@''PRAGMA_SYSTEM_HEADER''@|@PRAGMA_SYSTEM_HEADER@|g' \
+	      -e 's|@''PRAGMA_COLUMNS''@|@PRAGMA_COLUMNS@|g' \
+	      -e 's|@''NEXT_ASSERT_H''@|$(NEXT_ASSERT_H)|g' \
+	      < $(srcdir)/assert.in.h && \
+	  sed -e 's|_gl_verify|_gl_static_assert|g' \
+	      -e 's|_GL_VERIFY|_GL_STATIC_ASSERT|g' \
+	      < $(srcdir)/verify.h; \
+	} > $@-t && \
+	mv $@-t $@
+else
+assert.h: $(top_builddir)/config.status
+	rm -f $@
+endif
+MOSTLYCLEANFILES += assert.h assert.h-t
+
+Include:
+<assert.h>
+
+License:
+LGPLv2+
+
+Maintainer:
+Paul Eggert