build: fix the generation of the documentation.

Some of our targets use "bison --help", but they can't depend on
"bison" itself (to avoid additional requirements on the user), so
they used to call "make src/bison" in the commands.  Then
concurrent builds may fail: one make might be aiming one of its
jobs at compiling src/bison, and another job at generating the man
page.  If the latter is faster than the former, then we have two
makes that concurrently try to compile src/bison.

This might also be a more convincing explanation for the failure
described in the patch "build: fix paths".

	* Makefile.am (SUFFIXES): Initialize.
	* build-aux/move-if-change: New, symlink to gnulib's.
	* build-aux/local.mk: Ship it.
	* doc/common.x: Remove, merged into...
	* doc/bison.x: here.
	* doc/local.mk (doc/bison.help): New.
	($(CROSS_OPTIONS_TEXI)): Depend on it.
	Use src/bison.
	(.x.1): Replace with...
	(doc/bison.1): this explicit, simpler, target.
	(common_dep): Remove, inlined where appropriate.
	(SUFFIXES, PREPATH): Remove, unused.

doc/.gitignore

@@ -5,6 +5,7 @@
 /bison.cps
 /bison.dvi
 /bison.fn
+/bison.help
 /bison.html
 /bison.ky
 /bison.log