Add autogenerated dependency info to Makefile.

Adds a .depend file containing dependency information generated by
makedepend, which is appended to the generated Makefile by configure.

You can regen the file with "make -f Makefile.in depend" if necessary,
but we'll be looking at some way to automatically keep this up to date.

"no objection" djm@
diff --git a/INSTALL b/INSTALL
index 2289e80..f00b276 100644
--- a/INSTALL
+++ b/INSTALL
@@ -103,6 +103,13 @@
 FreeBSD 6.1 and OS X.  Alternatively, you may use the OpenBSM
 implementation (http://www.openbsm.org).
 
+makedepend:
+
+https://www.x.org/archive/individual/util/
+
+If you are making significant changes to the code you may need to rebuild
+the dependency (.depend) file using "make depend", which requires the
+"makedepend" tool from the X11 distribution.
 
 2. Building / Installation
 --------------------------