decouple zstd object files from lib/

decouple zstd object files from lib/

That was a subtle one :
VPATH is affecting search for both %.c source and %.o object files.
This meant that, when an object file already exists in lib/,
it's used in programs/,
even though programs/ is supposed to generate its own %.o object files.

With the new vpath directive, this is no longer the case :
the search is only activated for %.c source files.
Now, local programs/%.o are always generated
even if equivalent ones are already created in lib/.

It more clearly guarantees that lib/ and programs/ can use different compilation directives
without mixing resulting %.o object files.