Fix an autoconf issue with CLN in some configurations; also clarification re: license issues

[skip ci]
Merge branch '1.3.x'

[skip ci]
Printer support for GetProofCommand.
Fix bug 285; some cleanup.
Add support for Boolean variables, ITEs, and other things.
Proof-checking code; regressions now check proofs for all single-query valid/unsat regressions by default.