buildsys: move submodule to src, cleanups

buildsys: move submodule to src, cleanups

it's nicer as the build system is more coherent, i.e., I can do `make
install` from inside src and it actually works ;)

Use an atomic target for the build directory, else we can easily get
bogus builds.

Further use only one type of GNU make variable flavors, not both $()
and ${} mixed..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>