Allow lbtt not to be built, and skip relevant tests.
* m4/lbtt.m4: Turn the missing lbtt error into a warning, and do not configure lbtt wen --without-included-lbtt is specified. * bench/ltl2tgba/defs.in: Abort if lbtt is missing. * src/tgbatest/defs.in (need_lbtt): New function to skip tests that require lbtt. * src/tgbatest/babiak.test, src/tgbatest/ltl2neverclaim.test, src/tgbatest/spotlbtt.test: Call need_lbtt.