Check prerequisites in android.sh

dist-build/android.sh

@@ -1,10 +1,30 @@
 #! /bin/sh
 
+# Note that this script requires droid-wrapper
+if [ x`which droid-gcc` = x ]; then
+	echo "This build script requires droid-wrapper."
+	echo https://github.com/tmurakam/droid-wrapper
+	exit 1
+fi
+
+if [ -z "$NDK_ROOT" ]; then
+    echo "You should probably set NDK_ROOT to the directory containing"
+    echo "the Android NDK"
+fi
+
+if [ ! -f ./configure ]; then
+	echo "Can't find ./configure.  Wrong directory or haven't run autogen.sh?"
+	exit 1
+fi
+
 export CFLAGS="-Os -mthumb"
 export DROID_HOST=darwin-x86
+if uname -a | grep -q -i linux; then
+	export DROID_HOST=linux-x86
+fi
 export LDFLAGS="-mthumb"
 export NDK_PLATFORM=9
-export NDK_ROOT=/usr/local/Cellar/android-ndk/r8e
+export NDK_ROOT=${NDK_ROOT:-/usr/local/Cellar/android-ndk/r8e}
 export NDK_ANDROID_SOURCES="${NDK_ROOT}/sources/android"
 export TARGET_TOOLCHAIN_VERSION=4.4.3
 export TARGET=arm-linux-androideabi