only enable SHA backends that configure finds support for

doc/install.mdwn

@@ -16,7 +16,7 @@ To build and use git-annex, you will need:
   (or uuidgen from util-linux)
 * `xargs`: <http://savannah.gnu.org/projects/findutils/>
 * `rsync`: <http://rsync.samba.org/>
-* `sha1sum`: <ftp://ftp.gnu.org/gnu/coreutils/>
+* `sha1sum`: <ftp://ftp.gnu.org/gnu/coreutils/> (optional, but recommended)
 * Then just [[download]] git-annex and run: `make; make install`
 
 ([Ikiwiki](http://ikiwiki.info) is needed to build the documentation,

doc/install/OSX.mdwn

@@ -7,7 +7,7 @@ sudo port install pcre
 sudo cabal install pcre-light
 sudo cabal install quickcheck  
 
-# this will enable the gnu tools, (to give sha224sum etc..., it does not override the BSD userland)
+# optional: this will enable the gnu tools, (to give sha224sum etc..., it does not override the BSD userland)
 export PATH=$PATH:/opt/local/libexec/gnubin
 
 git clone  git://git.kitenet.net/git-annex