make: VERSION variable can be set either via git or the VERSION file

Change up the shell line so that it can fallback on the VERSION file
if building without a Git repository (eg, from a release tarball).

Additionally, change the titlepic generation so that it uses this
single instance of setting VERSION rather than doing its own thing.

graphics/titlepic/create_caption

@@ -1,7 +1,5 @@
 #!/bin/sh
 
-VERSION=`git describe` || VERSION=`cat ../../VERSION`
-
 convert titlepic.gif -fill white -font Helvetica-Bold                  \
   -draw "image over 94,2 0,0 '$1'"                                     \
   -pointsize 20                                                        \