[makefiles] Pass -Zlzma to dpkg-deb to force lzma compression.

The version of dpkg in Telesphoreo doesn't support the default in
newer versions of dpkg, -Zxz.
[makefiles] Set the THEOS_BUILD_DIR default to "debs".
[makefiles] I use GNU du, not the weird one.
[include] Use git://github.com/hbang/headers.git submodule as include directory.