android: Build and link with zlib

Previously, we linked with zlib shipped with NDK, but it seems this is
not part of NDK API, and thus could break our app.

README.rst

@@ -350,6 +350,7 @@ assumes the following libraries have been built for cross-compile:
 * c-ares
 * openssl
 * expat
+* zlib
 
 When building the above libraries, make sure that disable shared
 library and enable only static library. We are going to link those

android-config

@@ -55,13 +55,4 @@ PATH=$TOOLCHAIN/bin:$PATH
     CFLAGS="-Os -g" \
     CPPFLAGS="-fPIE" \
     LDFLAGS="-fPIE -pie -L$PREFIX/lib" \
-    PKG_CONFIG_LIBDIR="$PREFIX/lib/pkgconfig" \
-    ZLIB_LIBS="-lz" \
-    ZLIB_CFLAGS="-I$TOOLCHAIN/sysroot/usr/include" \
-    LIBSSH2_LIBS="-lssh2" \
-    LIBSSH2_CFLAGS="-I$TOOLCHAIN/sysroot/usr/include"
-
-# ZLIB_LIBS and ZLIB_CFLAGS are needed because aria2 configure script
-# checks zlib availability using pkg-config, but android toochain does
-# not provide pkg-config file. We need to specify these variables
-# manually.
+    PKG_CONFIG_LIBDIR="$PREFIX/lib/pkgconfig"

android/zlib-config

@@ -0,0 +1,22 @@
+#!/bin/sh -e
+
+if [ -z "$ANDROID_HOME" ]; then
+    echo 'No $ANDROID_HOME specified.'
+    exit 1
+fi
+PREFIX=$ANDROID_HOME/usr/local
+TOOLCHAIN=$ANDROID_HOME/toolchain
+PATH=$TOOLCHAIN/bin:$PATH
+
+HOST=arm-linux-androideabi
+
+CC=$HOST-gcc \
+AR=$HOST-ar \
+LD=$HOST-ld \
+RANLIB=$HOST-ranlib \
+STRIP=$HOST-strip \
+./configure \
+    --prefix=$PREFIX \
+    --libdir=$PREFIX/lib \
+    --includedir=$PREFIX/include \
+    --static