build: fix paths.

When using $(top_builddir) inconsistently, Make (including GNU
Make) is sometimes confused.  As a result it may want to build
lib/libbison.la and $(top_builddir)/lib/libbison.la (the same
file, different names) concurrently, which, amusingly enough,
might end with:

  ranlib lib/libbison.a
  ranlib lib/libbison.a
  make[2]: *** [lib/libbison.a] Segmentation fault

on OS X.

	* doc/local.mk, src/local.mk: Do not use $(top_builddir) when not
	needed.