use FORCE target as described in GNU make docs
add dist targets
simplify recursive build

... and allow LIBDIR, OPTIMIZATIONS to be specified
update zamautosat, zamvalve-tanh submodules
Merge branch 'master' of http://www.github.com/nphilipp/zam-plugins into nphilipp-master
Updated submodules

Signed-off-by: Damien Zammit <damien@zamaudio.com>