Factor out GC initialization code

This is not really part of the evaluator: it is just an integration
between Boehm GC and Boost coroutines usable for any purpose. The
evaluator (merely) optionally uses it.

src/libexpr/eval.hh

@@ -3,6 +3,7 @@
 
 #include "attr-set.hh"
 #include "eval-error.hh"
+#include "eval-gc.hh"
 #include "types.hh"
 #include "value.hh"
 #include "nixexpr.hh"
@@ -146,12 +147,6 @@ std::string printValue(EvalState & state, Value & v);
 std::ostream & operator << (std::ostream & os, const ValueType t);
 
 
-/**
- * Initialise the Boehm GC, if applicable.
- */
-void initGC();
-
-
 struct RegexCache;
 
 std::shared_ptr<RegexCache> makeRegexCache();