Use /bin/echo so -n works on osx.

The 'echo' built into /bin/sh on MacOS X doesn't support -n, which
confuses the AC_INIT macro. Use the separate binary which does.

doc/git-version.sh

@@ -10,4 +10,4 @@ else
   VERSION=${GIT_VERSION}
 fi
 
-echo -n ${VERSION}
+/bin/echo -n ${VERSION}