translate: relabel_bool was ignored when option_map was not supplied

* spot/twaalgos/translate.cc: Set relabel_bool_ to 4 by default, not
-1.
* NEWS: Mention the bug.

NEWS

@@ -7,6 +7,13 @@ New in spot 2.7.4.dev (not yet released)
     GraphViz that aborts when some label exceeds 16k characters, and
     also helps making large automata more readable.
 
+  Bugs fixed:
+
+  - spot::translator was not applying Boolean sub-formula rewritting
+    by default unless a spot::option_map was passed.  This caused some
+    C++ code for translating certains formulas to be noticeably slower
+    than the equivalent call to the ltl2tgba binary.
+
 New in spot 2.7.4 (2019-04-27)
 
   Bugs fixed:

spot/twaalgos/translate.cc

@@ -35,7 +35,8 @@ namespace spot
   void translator::setup_opt(const option_map* opt)
   {
     comp_susp_ = early_susp_ = skel_wdba_ = skel_simul_ = 0;
-    relabel_bool_ = tls_impl_ = -1;
+    relabel_bool_ = 4;
+    tls_impl_ = -1;
     ltl_split_ = true;
 
     opt_ = opt;