aria2: Add some build config options

aria2: Add some build config options

======================== ========================================
features                  dependency
======================== ========================================
HTTPS                    OSX or GnuTLS or OpenSSL or Windows
SFTP                     libssh2
BitTorrent               None. Optional: libnettle+libgmp or
                         libgcrypt or OpenSSL
Metalink                 libxml2 or Expat.
Checksum                 None. Optional: OSX or libnettle or
                         libgcrypt or OpenSSL or Windows
gzip, deflate in HTTP    zlib
Async DNS                C-Ares
Firefox3/Chromium cookie libsqlite3
XML-RPC                  libxml2 or Expat.
JSON-RPC over WebSocket  libnettle or libgcrypt or OpenSSL
======================== ========================================

Add 'CONFIG_' to 'PKG_CONFIG_DEPENDS'.

Signed-off-by: Hsing-Wang Liao <kuoruan@gmail.com>