By enabling wintls, we can use Windows certificate store to validate
server's certificate. Previously, we built windows build using
openssl and since we don't bundle CA certificates, aria2 fails to
validate server's certificate unless user setups their CA
certificates. GMP provides fast big integer calculations, whic is
used in BitTorrent encryption.
Not sure why PKG_CONFIG_LIBDIR does not work with mingw32 cross
compiler. Android toolchain works with it. As a workaround, we use
PKG_CONFIG_PATH for mingw32.