make fails with "cannot find z3"

[Michael Färber]

$ make
File "universo/solving/dune", line 4, characters 40-42:
4 |         (libraries dedukti.api checking z3)
                                            ^^
Error: Library "z3" not found.
Hint: try: dune external-lib-deps --missing @@default
make: *** [GNUmakefile:18: bin] Error 1

Since the integration of Universo, the README is outdated. It should probably mention that either
(a) one can avoid installing z3 if running something like make dkcheck.native, or
(b) make explicit that z3 is required, and which version of it.

[fthire]

I have made this dependency explicit in 8603294 but this a temporary fix. z3 should be optional.