android: add tips when SDK isn't configured.

clients/android/Makefile

@@ -1,7 +1,9 @@
 all:
+	./check-environment.pl
 	ant debug
 
 # Dummy target to make build.pl happy
 install:
+	./check-environment.pl
 	ant debug
 

clients/android/check-environment.pl

@@ -0,0 +1,14 @@
+#!/usr/bin/perl
+
+use strict;
+use FindBin qw($Bin);
+
+my $props = "$Bin/local.properties";
+unless (-e $props) {
+    die "\n".
+        "**************************************************************\n".
+        "Can't build the Camlistore Android client; SDK not configured.\n".
+        "You need to create your $props file.\n".
+        "See local.properties.TEMPLATE for instructions.\n".
+        "**************************************************************\n\n";
+}

clients/android/local.properties.TEMPLATE

@@ -0,0 +1,3 @@
+# Copy this file to one named "local.properties" and update this path to
+# wherever your Android SDK is located:
+sdk.dir=/home/bradfitz/sdk/android